(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "The movements and reactions of fresh-water planarians : a study in animal behaviour"

TRICLADiDA PALUDiCOU 



INVERTEBRATE ZOOLOGY 
SMITHSOMAN IMSTITUTiON 



ilP. i' 



New Series, No. 184 (Vol. 46, Part 4). 

FEBRIIAEY, 1903. 
THE 



Price 



QUARTERLY JOURNAL 



OF 



MICROSCOPICAL SCIENCE. 



EDITKI) HY 



E. RAY LANKESTEK, M.A., LL.D., E.R.S., 



SOKABV FELLOW OP EXETER COLLEGE , OXPOB D ; COH B ESPONDK > T OK THE INSTITUTE OK PBAXCI 

AND OK TUB IMPEBIAL ACADEMY OK SCIENCES OK ST. PETERSBURG, AND OK THE ACADKMV 

OK SCIENCES OK PUILADELPIIIA ; KORKIGN MEUBBR OK TUE BO»AL BOHEMIAN 

SOCIETY OK SCIENCES, AND OK TUE ACADEMY OK THE LINCEI OK BOMB, 

AND OK TUK AMERICAN ACADEMY OK ARTS AND SCIENCES OK BOSTON : 

ASSOCIATE OK THE ROYAL ACADEMY OK BELGIUM; HONORARY MEMBER 

UK THE NEW YORK ACADEMY OK SCIENCES, AND OK THE 

CAMBRIDGE PHILOSOPHICAL SOCIETY, AND OK THE ROYAL 

PHYSICAL SOCIETY OP EUINBUBOH ; HONORARY 

MEMBER OK THE BIOLOGICAL SOCIETY 

OK Paris; 

DIRECTOR OP TUB NATURAL HISTORY DEPARTMENTS OK THE BRITISH MUSEUM; LATE KULLERIAN 
PBOKBSSOR OK PHYSIOLOGY IN THE ROYAL INSTITUTION OK GREAT BRITAIN. 

WITH THE CO-OPERATION OF 

ADAM SEDGWICK, M.A., F.R.S., 

KELLOW AND TUTOR OK TRINITY COLLEGE, CAMBRIDGE; 

W. F. E. WELDON, M.A., F.R.S., 

LINACHE PROFESSOR OF COMPARATIVE ANATOMY AND KELLOW OK MEUTON COLLEGE, OXFORD: 

LATE KELLOW OF ST. JOHN's COLLEGE, CAMBRIDGE; 

AND 

SYDNEY J. HICKSON, M.A., F.R.S., 

BEYER PROtKSSOU OP ZOOLOGY IN THE OWENS COLLEGE, MANCHESTER. 

WITH LITHOGRAPHIC PLATES AND ENGRAVINGS ON WOOD. 




LONDON: 

J. & A. CHURCHILL, 7. GREAT MARLBOROUGH STREET. 

1903. 



Adliird and .Son,] 



[London and Dorkinc 



CONTENTS OF No. 184.-New Series. 



MEMOIRS : 

PAGE 

The Movements and Reactions of Fresh-water Plauarians : a Study 
in Animal Behaviour. By flAyMOND Peael, Ph.D., Instructor in 
Zoology in the University of Michigan, Ann Arbor, Michigan, 
U.S.A 509 -7||-, 

Ofl the Diplochorda. — Part IV. On the Central Complex of Cepliaifl- 
dl>aus dodecalophus, McI. By A. T. Mastekman, M.A., 
D.Sc, Lecturer on Zoology, School of Medicine, Edinburgh. (With 
Plates 33, 33) 715 

On Hypurgon Skeati, a New Genus and Species of Compound 
Ascidians. By Igerna B. J. Sollas, B.Sc.Lond. (With Plates 
34,35) 729 

The Anatomy of Arenicola ass i mills, Ehlers, and of a New Variety 
of the Species, with some Observations on the Post-larval Stages. 
By J. H. AsHWORTH, D.Sc. (With Plates 36, 37). . • -737 



y 



Title, Contents, and Index. 



iniuUiUsuH rauuui^uui \^ 



QUARTERLY JOURNAL 



MICROSCOPICAL SCIENCE. 



EDITED BY 

B. RAY LANKESTER, M.A., LL.D., P.R.S., 

ilONORARV FRl.I.OW Ol' EXETER COM.EGE, OXFOUD ; COR R KSPON nENT OF THE INSTITUTE OF FRANCE 

AND OF THE IMPERIAL ACADEMY OF SCIENCES OF ST. PETERSBURG, AND OF THE ACADKMV 

OF SCIENCES OF PHILADELPHIA; FOREIGN MEMUKR OF THE ROVAI. BOHEMIAN 

SOCIETY OF SCIENCES, AND OF THE ACADEMY OF THE I.INCEI OF ROME, 

AND OF THH AMERICAN ACADEMY OF ARTS AND SCIENCES OF BOSTON ; 

ASSOCIATE OF THE ROYAI. ACADEMY OF BELGIUM; HONORARY MEMBER 

<1F THE NEW YORK ACADEMY OF SCIENCES, AND OF THE 

CAMBRIDGE PHIl.OSOPHICAI. SOCIETY, AND OF THE ROYAL 

PHYSICAL SttCIETY OF EDINBURGH ; HONORAR Y 

MEMBER OF THE BIOLOGICAL SOCIETY 

OF Paris; 

nlHECTOR nr the natural history departments of the BRITISH MUSEUM; LATE FULLERIAN 
PROFESSOR OF PHYSIOLOGY IN THE ROYAL INSTITUTION OF GREAT BRITAIN. 

WITH THE CO-OPEUATION OF 

ADAM SEDGWICK, M.A., F.R.S., 

FELLOW AND TUTOR OF TRINITY COLLEGE, CAMBRIDGE; 

W. F. R. WELDON, M.A., F.R.S., 

LINACKE PROFESSOR OF COMPARATIVE ANATOMY AND FELLOW OF MERTON COLLEGE, OXFORD; 
LATE FELLOW OF ST. JOHN's COLLEGE, CAMBRIDGE; 



SYDNEY J. HICKSON, M.A., F.R.S., 

BEYER PROFESSOR OF ZOOLOGY IN THE OWENS COLLEGE, MANCHESTKK. 

VOLUME 46.— New Series. 
3i5litlj Ififljogrnpljit |!lales anb (L-ncirRbings on 3!5loob. 




LONDON: 
J. & A. CHURCHILL, 7, GREAT MARLBOROUGH STREET. 

1903. 



INVERTEBRATE ZOOLOGY 
SMITHSOMIAM mSTiTUTiOAl 



CONTENTS. 



CONTENTS OF No. 181, N.S., JULY, 1902. 
MEMOIRS : 

PAGE 

On a Free-swimmiiif^ HydroiH, Pelaf^ohydra mirabilis, n. £;en. 
et sp. By Arthur Dendy, D.Sc, F.L.S., rrofessor of Biology 
in tlie Canterbury College, University of New Zealand. (With 
Plates 1 and 2) . . . • • • -1 

Studies in the Retina. Parts III, IV, and V, with Summary. By 
Henry M. Bernard, M.A.Cantab. (From the Biological 
Laboratories of the Eoyal College of Science, London.) (With 
Plates 3— 5) 25 

Notes on the Relations of the Kidneys in Haliotis tuberculata, 
etc. By II. J. Fleure, B.Sc, U.C.W., Aberystwyth. (With 
Plate 6) '^^ 

Notes on the Development of Paludina vivipara, with special 
reference to the Urino-genital Organs and Theories of Gasteropod 
Torsion. By Isabella M. Drummond. (With Plates 7—9) . 97 

Is Chemotaxis a Factor in the Fertilisation of the Eggs of 
Animals? By A. H. Reginald Buller, B.Sc, Ph.D., Demon- 
strator in Botany at the University of Birmingham . .145 



CONTENTS OE No. 182, N.S., SEPTEMBER, 1902. 

MEMOIRS : 

Maturation of the Ovum in Echinus esculent us. By Thomas 
H. Bryce, M.A., M.D. (With Plates 10—12) . . .177 

Studies on the Arachnid Entosternite. By R. I. PococK. (With 
Plates 13 and 14) 



22J 



IV CONTENTS. 

PAGE 

On tlie Morphology of the Cheilostomata. By Sidney P. Harmer, 
Sc.D., F.U.S. (With Plates 15— 18) . . . .263 

On the Development of Sagitta ; with Notes on the Anatomy of 
the Adult. By L. Doncaster. (With Plates 19—21) . 351 



CONTENTS OF No. 183, N.S., DECEMBER, 1902. 

MEMOIRS : 

On a Cestode from Cestracion. By William A. Haswell, M.A., 
D.Sc., F.R.S. (With Plates 22—24) . . . .399 

The Development of Lepidosiren par ado xa. — Part III. De- 
velopment of the Skin and its Derivatives. By J. Graham 
Kerr. (With Plates 25—28) . . . , .417 

The Metamorphosis of Corystes Cassivelaunus (Pennant). 
By Robert Gurney, B.A.(Oxon.), P.Z.S. fWith Plates 
29"— 31) 461 

Artificial Parthenogenesis and Fertilisation : a Review. By 
Thomas H. Bryce ...... 479 



CONTENTS OF No. 184, N.S., FEBRUARY, 1903. 

MEMOIRS: 

The Movements and Reactions of Presh-water Planarians; a Study 
in Animal Behaviour. By Raymond Pearl, Ph.D., Instructor 
in Zoology in the University of Michigan, Ann Arbor, Michigan, 
U.S.A 509 

On the Diplochorda. — Part IV. On the Central Complex of 
Cephalodiscus dodecalophus, McI. By A. T. Masterman, 
M.A., D.Sc., Lecturer on Zoology, School of Medicine, Edin- 
burgh. (With Plates 32 and 33) ... . 715 

On Hypurgon Skeati, a new Genus and Species of Compound 
Ascidians. By Igerna B. J. Sollas, B.Sc.Lond. (With Plates 
34 and 35) 729 

The Anatomy of Arenicola assimilis, Ehlers, and of a New 
Variety of the Species, with some Observations on the Post- 
larval Stages. By J. H. Asiiwortii, D.Sc. (With Plates 36 
and 37) 737 

Title, Index, and Contents. 



fc>r, f 



-> 1 ' 

MOVKM?]NTS, ETC, OF riJESFI-WATER rr,AN A 1(1 ANS. 509 



';i>tx 



The Movements and Reactions of Freshwater 
Planarians : a Study in Animal Behaviour.' , 

By 

Kayiiioiid PcnrI, Pli.D. 

(rnsfniRlnr in Zonlotry in tiic Univcisily of Mirliipjan, Ann Arbor, 
Michigan, U.S.A.) 



Contents. 



A. Tntkodtjction . 

B. l^ESUME OF LlTERATUKE . 

I. Morplioiogical and Systematic 
JI. Pliy si logical 
c. Material 

D. Habits and Naturae History 

I. Occurrence and Distribution 
II. Activities 

a. Sensitivity 

h. Secretion of Mncus 

c. Periods of Activity and Rest 

d. Formation of Collections . 

e. Movement on Surface Film 

III. Food 

IV. Defecation 

V. Summary of Factors in JJcliaviour 

E. Normal Motor Activities 

I. Locomotor Movements 
a. Gliding 

1. Rate of Gliding Movement 

2. Direction . 




52G 
527 
527 
529 
532 
533 
534. 
535 
537 
538 
539 
539 
539 
545 
548 



' Contributions from the Zoological Laboratory, University of Michigan, 
Ann Arbor, Michigan, No. 58. 

VOL. 46, I'ART 4. NKW SERIES. LL 



510 



RAYMOND PEAKIi 



PAGE 

b. Crawl in J? Movement .... 548 

1. Direction . . . .550 

2. Stimuli which induce Crawlino: . . . 551 

c. Movement on the Surface Film . . . 552 
(1. Relation of the Movements of Tiiclads to those of other 

Forms . . . . • 553 

TI. Non-locomotor Movements . . . 555 

a. Contraction of the Body . . . ' . 555 

b. Extension of the Body .... 556 

c. Rest . . . . .557 

i. Formation of Collections . . . 566 

d. The Effect of Operations on Movement . . 570 
p. Eeactions to Stimuli ..... 576 

T. Reactions to Mechanical Stimuli . . . 576 

a. Methods . . . • .576 

b. Descrii)tion of Reactions .... 577 

1. Reactions to Stimuli applied to the Head Region . 577 

«. Reactions to Strong Stimuli . . 578 

j8. Reactions to Wc:ik Stimuli . . 582 

2. Reactions to Stimuli applied to the Middle Region of 

the Body . . . .588 

o. Reactions to Strong Stimuli . . 588 

jS. Reactions to Weak Stimuli . . 589 

S. Reactions to Stimuli applied to the Posterior Region 

of the Body . . . .592 

4. Reactions to Stimulation of the Ventral Surface . 594 

5. Reactions of Resting Specimens to Mechanical 

Stimuli . . . .595 

6. Reactions to Slimuli given by Operative Proce- 

dure ..... 595 

7. The Effect of Mechanical Hindrance to Movement . 597 

c. The General Features of the Reactions to Mechanical 

Stimuli . . . . .600 

f/. The Mechanism of the Reactions . . . 602 

1. The Relation of the Brain to the Reactions . 602 

2. The Neuro-muscular Mechanism . . 606 

e. Features in the General Behaviour of the Organism which 

the Reactions to Mechanical Stimuli explain . 619 

/. Summary . . . . .623 

II. Reactions to Food and Chemical Stimuli . . 623 

a. Food Reactions ..... 624 

1. Food Reactions of Specimens after Operations . 637 

2. Summary of Food Reactions , . . 640 



MOVKMKNTS, ETC., OF FEI']SH-WATRR TLANARJANS. 511 



b. Reactions to Chemical Stimuli — Cliemotaxis 

1. Reactions to Localised Chemical Stimuli 

a. Methods 
j3. Results 

2. General Summary 

3. Unlocalised Action of Chemical 
ITT. Thifjmotaxis and the Ri^htinj? Reaction 

a. Thiii;motaxis 

b. The Riifhtinj^ Reaction 

The ]\[echauism of the Reaction 

c. Summary 
TV. Elect rot ax is 

a. Methods 

b. liesults 

c. Mechanism of the Reactions 

d. Summary 
V. Tieaction to Desiccation . 

VT. Rlieotaxis 
G. General Summary and Di.scussion of Results 
H. List of Literature 



page 
643 
G43 
043 
646 
657 
669 
670 
670 
673 
676 
684 
685 
685 
685 
690 
695 
695 
697 
698 
7T0 



A. Introduction. 

The present study has for its purpose the analysis of the 
l)ehaviour of the coinniou fresh-water planarian into its com- 
ponent factors. It is well known that, aside from the 
researches of a few investigators on a small number of forms, 
we have little detailed knowledge of the behaviour of lower 
organisiiis. It is coming to be realised, too, tliat knowledge 
of what an animal does is just as important in the general 
study of life phenomena as a knowledge of how it is con- 
structed, or how it develops. But it must be admitted that 
until quite recent times the study of the activities of living 
things was a much neglected field in biology. The 
publication of the ' Origin of Species ' gave the biological 
pendulum a swing towards the study of phylogeny, from 
which it is onl}' just beginning to return. 

As a consequence of this concentration of interest on other 
subjects, we possess an accurate and full knowledge of the 



512 RAYMOND PRARL. 

activities of very few lower organisms. The beliaviour of the 
Protozoa has been quite fully described and analysed by tlie 
Avork of Verworn ('89) and Jennings ('97, '99, '99a, '99&, 
'99r, : 00, : OOff, : 00?^, :00c, : 01, Jennings and Moore :02). 
In the earlier work of Yerworn the general features of most 
of the reactions of the Protozoa are described, special atten- 
tion being paid to the rhizopods. The reactions of the 
Jiifnsoi-ia have been very thoroughly worked out by Jennings. 
In the case of the In-fusoria we now know exactly the 
mechanism of the renction to a large number of stimuli. The 
reactions and general behaviour in the case of two groups of 
echinoderms are quite thoronghly known from the early work 
of Preyer {'86, '87) on the starfish and the recent brilliant 
work of von Uexkiill ('96, '96ff, '99, : 00, :00ft) on the sea- 
urchin. These few instances are the only ones in the 
literature where the inovements and reactions of an organism, 
or group of organisms, have been investigated in any com- 
prehensive " monographic " way. There is a great body of 
literature dealing with isolated leactions in a variety of forms, 
but the thorough investigation of the activities of animals in 
a way comparable to that in which their morphology has 
been investigated remains in large degree yet to be done. 

It appeared highly desirable that this sort of knowledge be 
extended, and it was with this idea in mind that this work 
was undertaken. The form used, Planaria, was chosen for 
several reasons. In the first place, it has come to be a sort of 
paradigm for work (ui regeneration, and its biology from that 
standpoint is already well known. Furthermore, in some one 
or more of its species it is an almost universally distributed 
form and can always be obtained in quantities. Finally, and 
particularly, it is a representative of an animal type about 
whose activities we know only the most general facts. It is 
a symmetrical aquatic organism of low organisation, and its 
behaviour is rathei' complicated. The importance of possess- 
ing a detailed knowledge of the .activities of a bilaterally 
symmc'trical, free-moving, low organism will be apparent 
when it is considered that such an organism has never been 



MOVEMENTS, ETC., OF FI.'ESll-WATEi; PLANA IMANS. 513 

made the subject of sucli a study. Tlie behaviour of typically 
uusymmetrical organisms, the Infusoria, has been analysed, 
as has also that of some radially symmetrical animals, and in 
both cases there is found to be a very close iuterrelation- 
sliip between the general form of the body and the reactions. 

To investigate, then, iu a comprehensive way the activities 
of a bilaterally symmetrical organism standing low in the 
animal series was the purpose of this work. The most 
general problem which presents itself is the establishment 
of the animal's position in the objective psychogenetic series. 
Are its activities relatively simple or are they complex ? Do 
they fall under the same general type as those of the 
Infusoria or those of the higher organisms, or do they occupy 
an intermediate position ? Another general problem of im- 
portance is whether there is any marked correlation between 
the behaviour and the form of the body, such as has been 
found to obtain in so marked a degree in the case of the 
Infusoria and the rotifers (vide Jennings, loc. cit.). We 
have in the tlat-worm a symmetrical animal; are its reactions 
of a symmetrical type ? Besides these broad fundamental 
problems there are, of course, a large number of subsidiary 
questions which readily suggest themselves iu connection 
with a work of this sort. These need not be specifically 
mentioned here, but will be brought out in the course of 
the paper. 

As to the scope of the work as actually done, the following 
may be said: — The general '^ natural history" of the animal 
was studied as completely as possible. All the normal move- 
ments were studied iu detail. The reactions to mechanical 
stimuli; the food reactions and reactions to chemicals in 
general; electrutaxis ; thigmotaxis; rheotaxis; the righting 
reaction; the reaction of cut and regenerating pieces ; and 
hydrotaxis and the reactions during desiccation were investi- 
gated. No work was done on the phototaxis or thermotaxis. 
A study of the phototaxis was omitted for two reasons; first 
on account of the fact that during the progress of this in- 
vestigation Parker and Burnett (: 00) reported their work on 



514 KAYIMOND PEAllL. 

the sumo subject^ and furthermore on account of lack of 
opportunity. As a result of some incidental observations 
made during- the course of this work, it lias appeared that it 
would be profitable to extend the Avork of Parker and 
Burnett, and this, together with a study of the thermotaxis, I 
hope to be able to do iu the future. Another field for further 
work is afforded iu the study of the reactions of regenerating 
individuals. As this subject did not fall immediately into 
the general plan of this work, but comparatively little atten- 
tion has been g'iven to it, yet the work done gives much 
promise of important results to be gained by more extended 
investigations. 

So far as possible the details of the movements and 
reactions will be described fully. It is not easy to see why 
thei-e is not as much need for a complete knowledge of 
details in physiological work as in morphological, yet in 
much of the recent work in comparative physiology only the 
most general results are reported. To gain a knowledge of 
the details one must do the work over again. While such 
more or less general papers are easy to read, and put the main 
results in such a form as to be easily accessible, yet it is 
believed by the writer that the solid foundations of com- 
parative physiology and psychology must consist of detailed 
" fine " work, just as has been the case in morphology. It 
seems to the writer that the tendency to abandon the detailed 
descriptive method in favour of the extreme experimental 
method iu biological work is unfortunate. Both ways of 
working are methods of getting at the truth, and, as proven 
by their results, both are good methods. The current notion 
of the sufliciency of the experimental method to the exclusion 
of others is not only an evident exaggeration oi the facts in 
the case, but, in the opinion of the writer, the exclusive use 
of the " crucial-experiment " method in work upon the move- 
ments and reactions of organisms has in some cases hindered 
rather than hel[)ed us to gain a clear understanding of the 
])henomena. The importance of close observational work in 
the study of animal behaviour has been strongly emphasised 



MOVEMI'JNTS, KTC, Ob' FBE8H-\VATER I'LANAKIANS. 515 

recently by Wliittmm ('09). The aim in the present work 
has been to get as extensive and detailed a knowledge as 
possible of the behaviour of the organism by direct observa- 
tion before resorting to experiments. 

At this point I wish to acknowledge my indebtedness to 
the officials of the laboratory in which this work has been 
done. To Professor H. S. Jennings, under whose general 
oversight this investigation has been prosecuted, I wish to 
extend my heartfelt thanks for his uniform kindness in 
freely giving advice, suggestion, and kindly criticism of im- 
measurable value. Any adequate expression of my iudebted- 
ness to him is impossible. I further wish to express my 
thanks to Professor Jacob Reighard for the numerous 
facilities which I have enjoyed during my stay in his 
laboratory, and for his kindly interest, which has made work 
thei'e a pleasure. Finally, I desire to acknowledge my in- 
debtedness to Professor F. C. Newcombe, of the Botanical 
Department of the University of Michigan, for many 
valuable suggestions and advice. 

B. Resume op LrrERATURE. 

But little has been done on the physiology of the move- 
ments or on the psychology of the Turbellaria, and, as in 
the case of most of the literature dealing with these subjects, 
what has been done has been in comparatively recent years. 
Investigators of the old " natural-history " school which 
flourished before the time when Darwin's work changed the 
course of zoology seem not to have given much attention to 
planarians, while the later systematists and morphologists 
for the most part carefully avoided any reference to the 
activities of the forms which they studied. 

I. Morphological and Systematic. 

Among the papers devoted primarily to the systenuitic or 
morphological treatment of the group, there are occasional 
references to jioints in the behaviour of the oi-ganisms which 



516 RAYMOND PEARL. 

are of importance from the present standpoint. Aiuon 
such references the following may be noted : 

Moseley ('74), in a paper concerned principally with the 
anatomy and histology of the land plaiiarians, devotes a sec- 
tion to a discussion of the habits of these forms. He com- 
ments on the ''avoidance of light" (negative phototaxis) of 
hind and aquatic planarians, and discusses the habitat and 
food of the animals. He reaches the conclusion that all 
plauariaus are carnivorous, but gives no account of the 
method of feeding. He quotes Rolleston as having found 
that Planaria torva and Deudrocoelum lacteura in a 
dish in which had been placed a freshly killed earthworm 
" crowded on to the worm's body and soon sucked all the 
hajmoglobin out of it, leaving it white and pulpy." Brief 
mention is made of the habit of the land planarians of 
secreting a mucous thread and hanging from it as a mollusc 
does. Finally, the method of movement of Bipalium with 
the head raised and waved from side to side as the animal 
proceeds is described. A bibliography of previous literature 
is given. 

In another paper Moseley ('77, pp. 273, 271) gives an 
account of the movements and general habits of Geoplaua 
11 a V a, a Brazilian species. This species was found to keep 
in shaded and moderately lighted places. The direction of 
the ciliary currents was tested by placing small bits of paper 
on the surface of the body, and it was found that when the 
aninuil was in active movement the effective beat of the 
cilia on the anterior part of the dorsal surface was forward 
and outward, while on the posterior portion of the dorsal 
sui'face the beating was backward and outward. The 
currents on the ventral surface were always straight back- 
ward. The author concludes that the function of the cilia 
on the dorsal surface is to quickly remove foreign bodies. 
When the organism was at rest there was no movement of 
the dorsal cilia; "the animal moves to a large extent by 
muscular action, the body alternately contracting and ex- 
piinding during motion. When moving it lifted its anterior 



MOVEMENTS, EIC, OF FUESII-WATEU PLANARIANS. 517 

extfoinity offceiij and moved it to aud ffo as it" to fee 

or see its way/' " When the anterior extremity of the body 
was cut off the remainder of the animal seemed still to move 
with definite purpose, avoiding obstacles and retreating- from 
the light; while the cut end was raised and thrust in various 
directions as if to search for an object on which to climb." 

In a brief note Zacharias ('88) mentions the occurrence of 
Geodesmus terrestris between the lamellic of Agaricus 
deliciosus. Particular points mentioned are : the slow 
movement^ characterised by the raised anterior end, aud the 
hanging by a mucous thread after passing over the edge of 
a glass plate. Light stimulation of the anterior end with a 
needle induces a very strong contraction of the whole body. 

(Jamble ('03), in a systematic paper on marine 'i\xrbel- 
laria, describes briefly the movements of a number of 
species of rhabdocujles and triclads. 

Lang ('84), in his monograph on the polyclads, devotes a 
chapter to the habits, movements, and natural history of this 
group of })lanarians (loc. cit., pp. Goi — Gil). While not 
done particularly from the physiological standpoint and not 
treating the subject experimentally, this w(jrk contains 
numerous valuable observations. Points especially treated 
are the habitat, colouration, food and method of feeding, 
defecation, movements, including swimming, copulation, 
respiration, regeneration, growth, and duration of life. The 
details iu the behaviour of the polyclads recorded in this 
monograph will be discussed later in connection with the 
points on which they have direct bearing. 

The most important paper dealing with the movements 
and general behaviour of planarians which I Iiave been able 
to find in the literature is that of Lehuert ('91). This work 
is principally devoted to an account of the biology of three 
fornjs of land planarians, viz. Bipalium kewense, B. 
keweuse var. viridis, and Geodesmus bilineatus. 
Besides the work on these land forms, Lehnert also made 
some comparative studies on several fresh-water dendroceeles 
aud rhabdocoeles. He gives au admirably full and detailed 



518 RAYMOND PEAKL. 

account of the inoveiueuts of laud plauariaus; in fact, by far 
the best description of these phenomena in the Hterature. 
In this account the rehition of the movement to the mucous 
secretion from the ventral surface of the body is brought out 
in great detail. The principal factors in producing the 
movement in the case of the land planarians he gives as 
(a) ciliary movement on the ventral surface, [h] rhythmical 
contnictiou waves passing longitudinally over the ventral 
surface, (f) secretion of slime, and (d) snake-like movements 
of the whole body. A comparison with the movements of 
other planarians (fresh-water) is made. In this connection 
it may be mentioned that Lehnert considered rhythmical con- 
traction waves passing along the ventral surface of the 
animal to be a factor in the movement of fresh-water 
planarians (Dendrocujlum lacteum, PI an aria poly- 
chroa, and Polycelis tenuis). This I am unable to con- 
firm from observations on the planarians which I have 
studied. This point will be discussed more fully later. The 
food and the method of taking food in case of the land 
planarians, Lehnert worked out very thoroughly. They were 
found to be carnivorous, and in the case of Bipalium the 
pharynx was ca])able of being stretched over a large j)iece of 
earthworm, so that it resembled a very thin transparent skin 
covering it. 'JMie relations to other phases of the environ- 
ment, e.g. air, water, temperature, light, solid bodies, etc., 
are described very briefly. 

Easpail ('93), in a brief note, mentions the feeding of a 
planarian. 

Van Dnyne ('06) mentions briefly the movements of 
heteromorphic forms of PI an aria torva (?). He found 
that the parts of two-headed individuals moved inde- 
pendently of each other, and that each piece Avould move 
away from the other until they had completely torn apart. 

Willey ('07 j, in a brief note, describes the structure of a 
remarkable asymmetrical planarian, for which he proposes 
the generic name Hetero])lana, having the left side of the 
body almost completely atrophied. Regarding the loco- 



M0VI':MENT8, etc., of laiESll-WATEIi rJ,ANAUlANS. 519 

motion of this reiiuirkable form he says, '^lu llctorophma 

the locomotion is usually conducted in a somewhat 

one-sided fashion/' and he furthermore figures the animal 
as constantly moving* towards the left. It is to be regretted 
that no reference is made to how this form reacts to stimuli, 
as it would be of great interest to know whether the reactions 
are asymmetrical, and in general how they compare with 
the normal planariau type. 

A series of papers by Morgan ('08, : 00, : 01) contains 
numerous references of importance on the movements of cut 
and regenerated specimens of various fresh-water planarians. 
He finds ('98), in conlirmation of van Dnyue, that in two- 
headed individuals each head tends to move in its own proper 
direction. In the case of a hetex'omorphic form with two 
lieads pointed in opposite directions, this likewise held true ; 
but one component being stronger this determined the move- 
ment of the whole. The lack of movement in certain forms 
of cut pieces is also noted. In his : 00 paper Morgan notes 
the readiness with which " Planaria, sp. " ^ and Planaria 
maculata take food, although no account is given of the 
method of the feeding reaction. An interesting observation, 
and one of considerable theoretical importance, is also re- 
ported in tliis paper. In an individual split longitudinally in 
the median line from the posterior end forward, in which the 
two parts were united only by a small connecting band of 
tissue at the anterior end, it often appeared " as though these 
pieces would pull apart, but as soon as the tension on the 
connecting band becomes too strong, the rest of the piece, by 
a sort of adaptive response, ceases pulling in its former 
direction." In the most recent ])aper cited (:01) Morgan 
corrects a statement of Bardeen '-^ regarding the feeding of 
Planaria. It is maintained (and 1 may mention at this 
point that my own observations agree entirely with those of 
Morgan) that Planaria '^ responds freely" to food sub- 

1 Later iileiitiiied by Woodwoitli as I'l anuria lugubris. 
' To be reviewed later. 



520 RAYMOND PEAIIL. 

stances uot actually iu contact with it. This point will be 
discussed in detail later. 

Lillie (:01) bring's out the fact that cut posterior parts of 
the body of Dendrocoelum lucteum show very little 
movement^ and in general fail to give the typical reaction to 
light after removal of the brain. 

Filially^ there are accounts of the natural history and 
habits of various planarians in numerous "natural histories" 
and text-books. As such accounts are for the most part 
brief and of no great significance from our standpoint, they 
will not be referred to iu detail. 

II. Physiological. 

The literature dealing with the planarians from a purely 
physiological standpoint is very meagre. Furthermore, for 
the most part it deals only with special phases of the 
physiology of these organisms, there being very little work 
attempting to bring the behaviour of planarians into relation 
with that of other forms. 

The most important work dealing experimentally with the 
physiology of the movements of flatworms which I have 
found is that of Loeb ('Df). The purpose of his work was to 
determine in how far the reactions of such low organisms as 
worms were dependent upon the brain. The planarians used 
were Thy sano zoo n brocchii, and Planaria torva. In 
Thysanozoon he found that if the animal were quickly cut 
into two pieces transversely with a sharp scalpel or scissors 
the anterior piece crawled on undisturbed, while the 
posterior piece showed no further movement. The conclu- 
sion is then drawn that " Die Spontaneitiit der Progressiv- 
bewegungen ist also bei Thysanozoon eine Fuuktion des 
Gehirus." This form shows no definite ''geotropic " reac- 
tion, but crawls about with the axes of the body forming any 
angles with the direction of gravitation. The very strong 
" stereotropism " (thigmotaxis) of the ventral side of Thy- 
sanozoon, which always tends to keep in contact with a 
solid body, is noted. This reaction is found to be inde- 



MOVRMENTS, ETC., OF FRESn-WATER PLANARTANS. 521 

pendent of the lirain. There was fonud to be co-ordination 
l)et\veen the anterior and posterior pieces of a worm in 
wliich tlie Lateral Longitudinal nerves had been cut, but in 
which a narrow connecting strip of tissue had been left 
between the pieces. In P. torva Loeb states that posterior 
])arts of the bod}' which have been separated by a transverse 
cut fioni all connection with the brain crawl " ebenso ninnter 
weiter, wie die orale Halfte." The reaction of this form to 
changes of light intensity is discussed in considerable detail, 
it being shown that in strong light the organism is stimu- 
lated to active movement, while in the shade it remains quiet 
or moves very slowly. This was found to occur as well in 
decapitated as in nortnal woi-ms. The " stereotropic " reac- 
tion in this form is also mentioned. In concluding, the 
author holds that in worms there is no ''associative Gediicht- 
niss," and hence no consciousness. These results have been 
recently incorporated without essential change into a larger 
work (Loeb : 00). 

In an earlier paper Loeb ('9o) fii-st described the reactions 
to light of PI an aria torva. These results were incor- 
porated without essential change into the '94 pnper men- 
tioned above. 

Hesse ('97), in his morphological studies on the eyes of 
flat-worms, devotes a section to the subject of the reactions 
to light of Euplanaria gonocephala and Dendrocoelum 
lacteum. His results are confirmatory of Loeb's, nothing 
of particular significance being added. 

Steiner {'^S) found that posterior pieces of Planaria 
Neapolitana (=:8tylochus ])ilidium, Lang) separated 
from the brain by a transverse cut would move about freely- 
after recovery from the operation. He believes this ability 
to move is conditioned b}^ the presence of ganglion-cells in 
other parts of the body than the brain (along the lateral 
nerve-cords) . 

Parker and Burnett ( : 00) have recently made a thorough 
study, using very careful experimental methods, and treating 
the results statistically, of the reactions of Planaria 



522 EAYMOND PEART;. 

gonocepliala to light. This form moves away from the 
source of the light. The amount of directive influence was 
measured. It was found that specimens without eyes^ i.e. 
in which the anterior end had been cut off, react in much the 
same way to light as do normal individuals, 'Mn that they 
have a tendency to turn away from the course when directed 
towards the source of light, and to keep in it when directed 
awa}' from the source, though with less precision, and often 
to less extent, than planarians with eyes." Furthermore, 
figures are given showing that planarians from which the 
anterior end has been cut off move more slowly than normal 
animals. This is thought to bo due to the absence of the 
eyes. 

The most extensive paper dealing with the physiology of 
planarians is that of Bardeen (:01). This paper is mainly 
devoted to a stud}' of regeneration in Planaria niaculatn, 
but before enteriiig upon the discussion of this subject the 
author devotes considerable space to an account of the 
anatoni}'^ and physiology of the organism. In the section 
devoted to physiology, the author discusses, under the 
caption " Environmental Activities," sensation, movement, 
and the central nervous system. The author makes the 
remarkable, and obviously incorrect, statement that the 
planarian is sensitive only to light and contact. A ver}' few 
inconclusive experiments having reference to thigmotaxis, 
geotaxis (?) and hj-drotaxis, are reported. 'The statement is 
made that specimens " would remain unmoved by the presence 
close l)y their side of a piece of fresh snail, a food much 
prized by them." Two forms of movement are described — 
"swimming" and crawling. The author's description of 
Avhat he calls the " swimming " movement will be discussed 
later in this paper. Brief and very general statements 
regarding the reactions to mechanical stimuli are presented. 
Under the heading " Internal activities " are discussed 
deglutition, food dispersion, defecation, and respiration, in a 
rather loose and liy]iothetical Avay. Tiie author makes the 
followintj;; contribution regarding exci-etion in Planaria: — 



MOVKMKNTS, ETC., OF FRES[T-WATER PLANARFANS. 523 

''Excretion is carried in part through the intestines by the 
act of defecation ; in part it is doubtless carried on by an 
excretory system opening on the sui'face." A more detailed 
discussion of various points raised by Bardeen will be entered 
into in connection with the parts of this work on which they 
have direct bearing. 

A second paper by the same author (Bardeen^ : Ola) 
describes briefly the normal food reactions of Planaria, and 
shows that a decapitated specimen will not find food materinl 
in a dish, although one such a specimen could " be made to 
eat if it were placed on its back on a slide in a small drop of 
water. Under the conditions mentioned the pharynx is 
usually protruded, and will engulf bits of food placed in the 
mouth." An experiment was performed in which the pai't 
of the head in front of the eyes was cut off. Such specijnens, 
from which merely the tip of the head had been removed, re- 
acted normally to food. It is also shown that specimens 
fi'om which the part of the body posterior to the pharynx has 
been removed feed like normal worms. Regarding the 
method by which planarians find food in their immediate 
vicinity, Bardeen says (p. 176), ''It is difficult to determine 
the source of the impulse which gives rise to this purposeful 
activity. It is possible that the auricular appendages here 
act as delicate organs capable of stimulation by slight 
currents in the water set up by the minute organisms that 
prey at once upon the flesh of the dead snail." Experiments 
to be reported in the course of the present paper show, I 
think, that the mechanical and chemical stimuli given by food 
are the ones which affect planarians. 

c. Material. 

The following species have been principally used in this 
study : — Planaria maculata, Leidy ; Planaria gono- 
cephala, Duges ; Planaria dorotocephala, Woodworth.^ 
Of these P. dorotocephala and P. maculata have been 

' Excellent figures and descriptions of lljese three species have been pub- 
lislied by Woodwoitli, '97. 



524 RAYMOND PEAKL. 

most used, both on account of their abundance and, further- 
more, because P. dorotocephala is a form particularly 
favourable for the study of reactions. It is very active, and 
after being disturbed continues in movement longer than 
either P. macnlata or P. gonocephala, as has ali-eady 
been noted by Woodworth CIoc. cit., p. 7). I have found 
also that it moves faster than either of the other two species. 
There is a general precisiou and jiositiveness of i-esponse in 
its behaviour which make it es]iecia]ly favourable for experi- 
mental work. A large number of experiments have been 
made with a view to determining whether there was any 
difference in the i-oactions of these three species, but no 
essential difference has been found. The form of the reactions 
is the same in all cases. Whatever differences there are are 
differences of degree, such as would be conditioned by the 
relative sluggishness and activity. 

Certain forms of reaction to mechanical stimuli, and to 
chemical stimuli, are rather more easily induced in P. 
dorotocephala than in either of the others, yet, as will be 
shown later, these I'eactions will be given, under the proper 
conditions, by the other species. This being the case, and 
since P. dorotocephala was, for reasons noted above, most 
used in this work, it will be employed throughout the paper 
as the type form, and it will be understood, when there is no 
statement of the species, that P. dorotocephala is the form 
meant. 

No account of the anatomy of these forms will be given 
here, because it has been very fully treated in other readily 
accessible papers. The most important papers dealing with 
the morphology of the fresh-water triclads are those of 
Jijiina ('84), Lang ('81, '81a), Kennel ('88), Chichkoff ('92), 
and Woodworth ('91 and '97). 

Besides the species mentioned above, on which the most of 
the work was done, a number of observations and experi- 
ments have been made on several other species of triclads 
and rhabdocoeles. The other triclad most frequentlj' met, 
and whose reactions have been found to agree closely Avith 



MOVEMENTS, ETC., OF FRKSH-WATER PLANAllIANS. 525 

tliose of tlio species of Plauaria, is a form Avliich agrees 
witli tlie description of Dendrococluni lacteum, except in 
respect to the colour. This form is usually coloured from a 
light grey to nearly black. The colouring is uniform. In 
only one specimen have I found any deviation from this 
typical coloration, and in that case there Avas a band of 
black pigment extending the whole length of the body 
along the mid-dorsal line. In width tliis band occupied about 
one third the whole width of the body. The margins were a 
pure white, without the faintest trace of pigmentation. This 
specimen was kept under observation for some time, and 
there was no doubt that it belonged to the same species 
as the grey form. The specimen struck one at once as being- 
transitional between the ordinary white to cream-coloured 
Dend rocoelum lacteum, and the grey form found about 
Ann Arbor. Being in some doubt as to the true taxonomic 
position of this grey species, I shall refer to it throughout 
this paper as Dendrocoelum, sp. 

Besides the forms mentioned, several undetermined triclads 
have been collected and worked with to some extent, but as 
no new factors presented themselves in their reactions they 
will not be considered in this papei'. 

Alargei'habdocoele, which I have identified asMesostoma 
personatum, 0. Schm., is found rather commonly in certain 
localities about Ann Ai-bor in the spring. I have done some 
woi'k on this form. Another rhabdoco?le whose actions I 
have studied to some extent is Stenostoma leucops, O. 
Schui. No detailed investigation of the behaviour of the 
vhabdocosles was made, Init as opportunity offered they were 
used for comparison. 

The methods used in experimentation will be given under 
the separate headings dealing with the reactions. 

T). Habits^ and Natural History. 
In the course of more tlian two years' study of planarians 

' In tills section the word " habit " will be used to signify merely tliose 
activities of the organisms which are frcquontly ol)served to occur under 
VOL. 46, PART 4. NEW SERIES. M M 



526 RAYMOND PEARL. 

numerous observations have been made on tlieir general 
natural llistor3^ It is thought desirable to present a general 
account of this here for two reasons : first, because there is no 
adequate discussion of the natural history of the fresh-Avater 
triclads in the literature; and furthermore, because it will 
bring out prominently the phenomena for which we are 
seeking an explanation. In other words, it will present the 
problems with which this study has had to do. 

I. Occurrence and Distribution. 

The species of Planaria (maculata, dorotocephala, and 
gonocephala) used in this study have been collected mainly 
from the Huron River near Ann Arbor. They are found, for 
the most part, on the under surfaces of stones in places where 
the current is of moderate swiftness, and the substrate on 
which the stones rest is rather soft. They are also found 
among the fronds of such water plants as Ceratophyl lum 
and El odea, although less abundantly than on stones. I 
have obtained these species only very I'arely in collections 
fi'om ponds and small pools of stagnant water. They appear 
to be, in general, much more abundant in shallow water than 
in deep. 

Rhabdocoeles I have found in great abundance in small 
ponds and pools of stagnant water, and, with the exception 
of Stenostoma leucops, almost never in running water. 
Dendrocoelum, sp., is also much more abundant in stagnant 
Avater than in streams. 

There is no marked seasonal distribution of the species of 
Planaria studied. They appear to be slightly more abun- 
dant in the fall than in the spring. I have found no evidence 
of any migration into deep water during the winter in the 
case of these forms, as has been described by Child (: 01, pp. 
978 — 981) as occurring in Stichostemma. The seasonal 
distribution of Dendrocoel um,sp., appears to be well marked, 

iiat.ui-al conditions, williout necessarily implying Uie same idea as that embraced 
in tlie term " habit " as used by the psychologists. 



MOVEMENTS, ETC., OF PHESH-WATKB PLANAEIANS. 527 

individuals being found in considerably greater numbers in 
the spring than at any other time in the year so far as my 
observations go. This seems to be true also of the rhab- 
docoeles. 

Relatively the most abundant species of planarians in this 
region aie Planaria dorotocephala and maculata. The 
numbers of these two are about equal, with a slight advantage 
in favour of P. dorotocephala. Next in abundance I have 
found to be P. gonocephalaj but this is considerably below 
the other two. Finally comes Dendrocoelum, sp., which I 
have never found in an abundance to be compared with the 
species of Planaria. 

II. Activities. 

The movements of planarians will be discussed in detail in 
a later section of the paper,^ but it is desired to take up here 
certain general activities and relations to the environment 
which properly fall under the general subject of natural 
history. 

The first of these subjects is — 

a. Sensitivity. — The flat-worm is extremely sensitive to 
a variety of stimuli. Among the different stimuli which pro- 
duce specific reactions, and to which we must therefore 
conclude it is sensitive, are the following : — Mechanical 
disturbances of the general environment (shaking, jarring, 
movement of Avater, etc.), contact (localised mechanical 
stimulation), chemical changes in the environment (in the 
widest sense, including food substances), light, the electric 
current, desiccation, a current of water, and heat. 

Its extreme sensitivity, which makes it responsive to very 
slight changes in the environment, mny bo shown by a very 

' It may be slated lieie, for the conveuience of llie reader before reacliinjf 
the full discussion of the movements, that the progressive movements of 
triclads are of two sorts. These are (a) gliding movements, in which there is 
little or no muscular action ; and (6) crawling movement?, in which the motion 
is effected by muscular contractions involving the wiiole body. Tlie crawling 
has some general resemblances to the method of progression observed in a 
leech of the genus Clepsine. 



528 EAYWOND PEAEL. 

simple experiment. If a dish containing specimens not m 
any wa}^ stirred up by rough handling, but gliding along the 
bottom, be jarred ever so slightly, every individual will 
instantly stop, contract, and remain immovable. If only a 
single jar is given, the worms will start after only a momentary 
pause. A further experiment shows more strikingly the same 
thing. If in a dish containing water to a depth of not more 
than 1 to 1-5 cm. a single specimen gliding quietly is 
selected, and a needle is touched to the surface of the water 
above or to one side of it, it will be seen, if closely watched, 
to give the same momentary pause and partial contraction. 
If the needle is pushed down through the water towards the 
worm in any but the quietest and gentlest way the contracted 
state will continue. Only at the moment when quietness in 
the surroundings intervenes again will movement be resumed. 
I have frequently tried to introduce a needle close beside the 
animal without causing this momentary pause. With a layer 
of water not over a centimetre in depth covering the worm I 
have not been able to do this, except in rare instances. 
After the point is once through the surface film it may be 
brought nearer the worm without causing a persistence of the 
contraction, provided it is advanced in line with itself, i. e. 
not slid up laterally. In order to observe this extreme sensi- 
tiveness to disturbance of the water one must take care that 
the animals have not been violently disturbed just previously. 
Any marked disturbance or persistent, more or less violent 
stimulation puts the animals in a condition which may be 
called, for lack of a better term, " excited." Such a condition 
is characterised by increased rapidity of movement and in- 
creased general activity, and in this condition the animals do 
not give the "■ finer " responses, — that is, responses to weak 
stimuli. I shall have occasion to discuss this matter in more 
detail later. 

This marked sensitivity and its associated behaviour are 
remarkably similar to Avhat has been found by Whitman ('99) 
to obtain in the case of the leech Clepsine. He has further 
pointed out that lack of attention to this extreme sensitivity. 



MOVEMENTS, ETC., OF EKESH-WATEIi PLANAHIAN.S. 529 

which is apparently quite generally distributed aniono- lower 
organisms, may be an important source of error in work on 
the behaviour. 

Regarding the statement of Bardeen (: 01, p. 14) that he 
does not find that Planaria " is sensitive to anything but light 
and contact," nothing need be said here. The detailed 
accounts of the reactions of the organism to a variety of 
stimuli which iollow in this paper are in themselves a 
sufficient criticism. 

h. Secretion of Mucus. — There is secreted at all times 
over the surface of planarians a sticky slime, apparently of 
the nature of mucus. This secretion is increased when the 
animal is irritated, and is under normal conditions more 
abundant on the ventral than on the dorsal side. If a 
needle or fine glass rod is touched several times on the 
surface of the body its end becomes covered with this 
secretion. For this reason it is necessary in applyin<)^ 
localised mechanical stimuli to wipe the mucus off the end 
of the needle frequently, in order to obtain good results. 
Similarly, iP one is using a sharp scalpel to cut the animals, if 
the edge is left in contact with the surface of the body an}' 
length of time before the decisive cut is made, the edge will 
become so coated with mucus that a clean cut is impossible; 
instead, the animal will slip from under the knife. 

When the animal moves about it leaves behind a more 
or less heavy string of this mucus, so that if several speci- 
mens are placed iu a clean glass dish the bottom will, in 
a short time, become covered with a network of interlacing 
mucus threads. The same phenomenon occurs in other 
Turbellaria and among the Nemerteans (cf. Child, : (Jl, 
and Wilson, : 00). The threads when first secreted are so 
transparent as to be invisible, but in larger quantities they 
appear opalescent, and may be picked out of the dish with 
forceps. 

The function of this secretion iu locomotion is evidently to 
attach the body to the substrate. Secretions for such a 
purpose occur widely among aquatic organisms. 



530 RAYMOND PEARL. 

The mucus also undoubtedly plays au important part in 
the attachment of the animal to the under side oE the surface 
film. When the worm leaves the surface film in open water, 
i. e. when it cannot reach any solid body, it hangs by the 
mucus thread in much the same way that a terrestrial 
mollusc, like the common slug, does when it passes through 
the air from a higher to a lower point. This observation I 
have made many times, though generally in an indirect way. 
As has been said before, the mucus thread is invisible when 
first secreted, so that when a worm leaves the surface film it 
seems to glide freely through the water. If, however, one 
passes a needle horizontally through the Avater immediately 
above the posterior end of a worm which has just left the 
surface film, it will be seen that at a certain point (where the 
needle strikes the thread) the end of the worm will be jerked 
to one side. Furthermore, one may with care pick up the 
invisible mucus thread with forceps and raise the whole 
worm, provided the attempt is made before the anterior end 
reaches the bottom. I have seen specimens of P. maculata 
craAvl back upon the thread after going a part of the way 
down to the bottom, and again regain a position on the 
surface film. The same thing is frequently done by slugs. 
When the animal has fully reached the bottom, connection 
with the thread which has served to suspend it in the water 
is usually broken by several sharp jerks of the posterior end 
of the body from side to side. 

The relation of the organism to this slimy secretion is 
much the same in the land plauarians, according to the 
observations of Lehnert ('91). He distinguishes in case of 
these forms " Kriechfaden,^' " Briickenfaden,'' and " Gleiten- 
faden " formed fi'om the slime, the distinction being based 
on the relation of the thread to the surroundings. The 
" Kriechfaden " are the threads left behind as the organism 
moves over a continuous solid body, and the " Gleitenfaden " 
are the threads on which the animal hangs in passing 
throuo-h the air from a hio-her to a lower level. Both these 
forms of threads I have found in case of the fresh-water 



MOVEMENTS, ETC., OF FRESn-WATER I'LANAIilANS. 531 

plauarians. The " Bruckenfadcn," wliicli are formed by the 
hiiid plauarians when they pass from one solid body to 
another at about the same level, I have never observed in 
case of fresh-water planarians, tiiough I see no reason why 
under proper conditions they would not be formed. Lehnert 
(loc. cit., p. 17) says, " Die Wasserplanarien bilden wie die 
Landformen ihren Kriecli-, Briicken-, und Gleitfaden." He 
also noted that Poly cells tenuis was able to crawl back 
upon a mucus thread after passing for some distance down 
over it. 

Nothing- like the formation of "cysts" froui this mucus, \ P^'^ 
such as Child (: 01, pp. 989 to 993), found in the case of 
Stichostemma, has been observed in the case of planarians. 
Its only biological significance in these forms is in relation 
to movetuent, as pointed out above. 

In connection with the subject of mucus secretion it may 
be well to point out the tenacity of the attachment of the 
fiat-worm to the bottom. It will be found in attempting to 
dislodge the animal that the extreme anterior end and the 
extreme posterior end stick very firmly to the substrate. 
Whether this holding is the result of a sucker-like action of 
the ends of the body, or is due merely to the stickiness of the 
mucus, I have been unable to decide. It is easily possible 
that the muscles could be so contracted as to form out of 
either end of the body a practical sucker, but whether this is 
done or not it is impossible to say. Woodworth ('97) has 
described a permanent anterior adhesive disc in Dendro- 
coelum lacteum, but considers that this "is not a true 
sucker, nor does the animal employ its anterior end for the 
purpose of attachment to any greater degree than the 
posterior or lateral margins of its body, along the ventral 
surface of which numerous mucus glauds have their 
openings. In truth, it is the margins and posterior end that 
adhere more firmly to a support ; often when the animal is 
forcibly removed from the sides of the aquarium the parts of 
the margin or the posterior end will adhere so firmly to the 
glass that the points of attachment are drawn out into 



532 KAYiMOND PEARL. 

digitate processes." I incliue to the view that in Plan aria 
ib is the mucus which attaches the organism to the support, 
although it must be said that the jippearance is at times 
strikingly as if the anterior and posterior ends acted as 
suckers. 

c. Periods of Activity and Pest. — There is in the 
case of freshly collected planarians a certain periodicity in 
the activities. First, there is the rather marked difference 
in the amount of activit}^ in the night aud day. It has been 
stated by a number of investigators that planarians were 
probably nocturnal in their habits, i. e. more active at night 
than during the day. This can easily be seen to be the case 
in the following way : — In a dish containing a large number 
of planarians, together with some plant material like Cera- 
tophyllum, usually comparatively few specimens will be 
seen during the day. Nearly all will be in among the fronds 
of the plant material in a quiet condition. If, however, one 
comes into the laboratory at 8 p.m. or later at night, so that 
(in case of winter days) there has been two and a half or 
more hours of darkness, a large number of the specimens 
will be found on the sides and bottom of the dish in active 
movement. Again, one will frequently find in the morning 
that the specimens are scattered about all over tlie sides 
and bottom of the aquarium dish at rest. By noon numy 
of these will have disappeared, or, in other words, gone 
in among the plants, where they are protected from the 
light. 

Besides this day and night periodicity there is another 
fact that may be mentioned; this is that during the day, 
at any rate, they seem to be incapable of continuing 
movement more than a certain, not very great, length of 
time. Then a period of rest must intervene. Thus one may 
see a specimen which has been moving about come to rest, 
and after a length of time, varying from a comparatively few 
minutes to several hours, it Avill start into spontaneous move- 
ment again, and repeat the Avhole cycle over and over. It 
seems that the periods of ({uiet are really for the })urpose of 



MOVKMKN'I'S, E'I'O., OF FRKSll- WATKi; I'l.ANAK lANS. 533 

resting, i. e. the aniiua-l seems to ])e (luickly fatig-ued by its 
own movements. Tin's is indicated by the fact that it' one 
stirs up a specimen, and sets it into activity again just as 
soon as it comes to rest, the periods of spontaneous activity 
will become progressively shorter, until finally the wcn-m will 
only move a very short distance before coming to rest again. 
The periods of activity are longer and more frequent in 
P. dorotocephala than in any of the other species I have 
studied. 

d. Formation of Collections. — Tlu'rt; is a well-marked 
tendency for specimens of planarians to form well-defined 
groups or collections when they come to rest on an open 
surface like the bottom or sides of a glass dish, or on the 
under side oC rocks, under natural conditions. Of course. 



C.^ 



I'lG. 1. — Diagram bliowiiig I he apjjcaraiice of a culled ion of ifbUiig 

jilaiiariaiis. 

this is in part a result of tlieir reaction to ligld, as has been 
noted by Loeb ('04). Besides this there seems to be some 
other factor at work, for in the same dish one fre([uently 
finds several localised collections from one to two inches in 
diameter in different parts of the dish. In these collections 
the specimens may be closely packed t(.)gether, and with 
some specimens overla]iping and lying partly over others, yet 
in the species I have studied a looser arrangement of the 
character shown in fig. 1 is the more usual one. On the 
under surface of stones such groups arc frequently seen ; 
two or three nniy bi^ ft)uud on the same medium-sized stone. 
In this case light as a factor cannot be present, since the 
couditions of all with reference to this stimulus are equal. 



534 RAYMOND PEARL. 

We have, then, here a case of what appears superficially to 
be " social instinct." 

e. Movement on Surface Film. — As is well known, 
flat-worms and a number of other animals frequently move 
about on the under side of the surface film at the top of the 
water. On account of the flexibility of the support, motion 
under these conditions is very slow, and usually, after having 
been on the surface film for a short time, the worm will 
loosen its hold and pass down to the bottom in the way which 
has been described above. The worms do not remain 
customarily in the angles formed by the surface film with the 
side of the dish, as does Stichostemma (Child, :01), but 
instead pass out at once on to the free surface. Further, the 
flat-worms never push through the fllm at the side of the 
dish and pass up out of the water as the nemertean does. 
The occurrence of planarians on the surface film is not the 
result of any thigmotactic reaction (using thigmotactic in the 
sense ordinarily understood), but is brought about by a 
simple reflex act, and is tlie result of the configuration of the 
surface of the water and the side of the dish. This will be 
brought out in more detail later. It is probable that fresh- 
water planarians, in their normal habitat, very rarely take up 
a position on the surface fihu. Among other organisms 
(Entomostraca, Hydra, etc.) this habit probably has a much 
greater biological significance than in planarians (cf. 
Scourfield, 'Of, : 00, :01). When on the surface film the 
worm behaves in nearly every respect as it does when on the 
bottom. The head is frequently raised (with reference to 
the worm) and waved about in the water just as occurs in 
the normal movement. That the situation is a more or less 
abnormal one, however, is shown by the fact that, so far as I 
have observed, the worni never comes to rest on the surface 
film, but instead, always keeps in active movement till it 
leaves it. 

The means by which the animals maintain their position on 
the under side of the film is undoubtedly the mucous secre- 
tion from the ventral surface. This is very sticky, and holds 



MOVEMENTS, ETC., OF FRESU-W/Vl'ER PLANA HIANS. 535 

tlio animal to the film, the surface tension being sufficiently 
great to support a considerably greater weight than that of 
a flat- worm. 

It is interesting to note in this connection that the land 
planarians are able to move about on the top of the surface 
film of water to a limited extent (cf. Lehnert, loc. cit., 
p. IG). The immediate means of support here, as in the case 
of the fresh-water planarians, is the mucous secretion. 

The leaving of the surface film by means of the mucus 
thread described above apparently does not take place if it is 
possible for the same result to be accomplished in any other 
way. Before it occurs the worm usually stretches the 
anterior end down into the water, and turns it in all direc- 
tions. If it comes in contact with something solid the 
anterior end becomes attached to this, and pulls the posterior 
end of the body away from the film. If nothing solid is 
within reach the worm Avill usually, after a time, drop down 
on a mucus thread as described. 



III. Food. 

Planarians will take almost any sort of animal food very 
readily, I have used mainly, in the feeding experiments, 
crushed pieces of fresh-water molluscs, such as Physa, 
Planorbis, etc. One of these molluscs, removed from the 
shell and placed in a dish centainiug a large number of 
planarians, will, in a short time, be literally covered with the 
worms feeding. If a worm is gently lifted off the ])ile the 
greatly stretched pharynx will bo brought into view. The 
worms will eat any other kind of animal tissue (fresh meat, 
parts of insects, pieces of fresh-water worms, etc.), so far 
as I have observed, tlu; only condition being that the meat 
must be fresh. As will be shown latei", the juices from the 
food act as chemical stimuli, so that it is necessary that the 
tissue be crushed or bruised so that its juices can escape into 
the water. A pai'tially crushed specimen of PI an aria, even 
though still able to move about, will be seized upon and eaten 



536 RAYMOND PEAlil;. 

as quickly as any other food. I liavu several tiiues seeu 
specimens thus eaten. It is, in fact, possible, with a little 
patience, to make a specimen eat a small piece cut olf the 
posterior end of its own body ! This eating of each other 
does not occur, so far as I have observed, unless an individual 
is bruised so that some of the tissue underlying the epidermis 
is exposed. Under these conditions juices escape from the 
body and act ;is stimuli on the other worms. Under normal 
conditions contact of one individual with another does not 
start the feeding reaction, Avhich is a purely reflex pheno- 
menon, cispablo of being started only by a certain set of 
stimuli. Promiscuous cannibalism, such as Child (:Ul) 
suspects to occur among individuals of Stichostem ma, I 
have seen no evidence for among the Turbellaria.^ 

In the feeding the worm lies fully 'distended, with the 
posterioi' two thirds of the body on the meat, or whatever 
else is being used for food ; the pharynx is extruded, 
frequently to nearly half the length of the body, audits end is 
attached to the meat. During the feeding the very anterior 
end of the worm is attached to the bottom of the dish, provided 
the piece of food is not so large as to make this impossible. 

Besides the animal food which the worms will take so 
readily, they also normally, probably to some extent, feed on 
vegetable mattei', although I have not been able to induce 
the typical food reactions (to be discussed later) with vege- 
table material. The evidence for the statement that 
vegetable food is used by planarians is of two sorts : (o) 
specimens a,re frequently found extended on the stalks of 
water plants, with the pharynx extruded and attached to the 
stalks; and [b) the faeces which have been observed immedi- 
ately after defecation have been found to consist largely of 
finely divided plant tissue. It would appear, however, that 

' Biirdccu (: 01, A, p. 176) says, "Stroiii,^ planaiiaiis oficu \n-c.y upon weak 
ones. Ill such instances the strong individual al laches its pharynx somewliere 
upon the boi]y of tlie weak one, usually near tiic liead." I iiavc never seen 
even the largest specimens eat smaller ones unless these latter were bruised 
in some way. 



M(WrL\[KN'l'.S, KTC, OF FRRST[-\VATKIi PLANARTAXS. 53/ 

veg'ofcable food is not alone sufficient to keep tlie animals in 
good condition^ for specimens kept in an aqnarinm dish with 
plenty of living plant material, on ■which they stay the 
greater part of the time dni'ing the day, will steadily grow 
smaller niiless animal food is given them. 

The food is in part digested, or at an}' rate softoiied, and 
physically changed ontside the body. A piece of mollusc, 
on which a nnmber of worms have been feeding for some 
time, has a white, Huffy appearance, similar to that of meat 
after partial gastric digestion. 'J'his is ajiparently brought 
about by a secretion poured out of the end of the pharynx. 
The necessity for some such action is apparent, because the 
flat-worm has no teeth or other means of separating a portion 
of ordinary tough fibrous tissue off from a mass so that it can 
be swallowed. This can be done, liowever, if the mass is first 
softened and partially dissolved. There are certain other 
evidences that a secretion is poured out from the pharynx 
during the feeding process. These will be taken up in 
ano-ther connection. 

After the worms have fed undisturbed for a certain length 
of time they will leave the meat, and, after a short period of 
activity, come to rest. 

The worms are able to live for a considerable length of 
time (at least two months) without food, although the}- 
continually grow smaller dui-ing this time. This marked 
decrease in size while starving has been noted by several 
observers, and especially studied by Lillie (:00). This 
author finds that the decrease in size is accompanied by a 
simplification of structure — a sort of "development back- 
ward," such as has been described by Patten ('06) for ab- 
normal embryos of Limulus. 

IV. Defecation. 

The process of defecation has been observed by Bai-deen 
(:0l). The process consists of three or four general con- 
tractions involving the whole body, during which the 



538 RAYMOND PEARL. 

coutents of the intestine can be seen to be in rapid motion. 
Soen after the beginning of the contractions, Avhich are in 
character different from any other of the movements of the 
body which I have observed, and which cannot be adequately 
described, the intestinal debris is shot out of the pharynx. 
The force of the expulsion is so great that the f^ces spread 
out in the water some considerable distance from the opening 
of the pharjaix. I have observed the process only a few 
times ; apparently it occurs only at infrequent intervals. 

V. Summary of Factors in Behaviour. 

From the above sketch the behaviour of the flat-worm can 
be seen to be of considerable complexity. The movements 
show many variations in character, rate, and direction. The 
animal shows apparent preferences for certain situations 
while avoiding others. It reacts to a variety of stimuli in 
ways which, on the whole, further its preservation and well- 
being perhaps as well as if guided by careful thought. It 
chooses its food, taking certain things and passing by others. 
It forms gatherings of a sort which apparently indicate that 
the flat-worm prefers to be in the company of his fellows ; 
in other words, it seems to have something of "social 
instinct." On the whole, as the further analysis will show, 
it fits itself to its environment by its activities in a way 
which would not be discreditable to a being possessed of 
considerable powers of reasoning. 

Our problem now is to analyse, as far as possible, this 
complex behaviour into its component factors. Each activity 
will be taken up in detail and subjected to thorough scrutiu}', 
to determine its essential nature, and whether it may not be 
resolved into simple components. With this analysis com- 
pleted, it will be possible to assign the organism a definite 
position in the objective psychological scale. With the 
internal psychological factors — those of which there is no 
objective criterion — we shall not attempt to deal. The 
purpose of the paper is to furnish the data which may be 



MOVRMENTS, ETC., OP FRKSH-WATKR TLANARIANS. 539 

obtained by an objective study of the phenomena : precisely 
what these imply as to internal factors would doubtless be a 
subject of dispute among psychologists of different schools. 

E. Normal Motor Activities. 

Under this heading will be included all the purely motor 
phenomena of the organism. This will include the 
movements (without refei-ence to special reaction to stimuli), 
the coming to rest^ and the general resting condition of the 
organism. 

The movements natui*ally fall into two categories; (a) 
locomotor movements, and [h) non-locomotor, including such 
movements as contractions and expansions and the like. 

I. Locomotor Movements. 

As has been mentioned above (p. 19), thei-e are two sorts 
of locomotor movements, the gliding and the crawling. The 
gliding is the smooth, even motion by which the flat-worm 
slips about over surfaces Avithout showing a perceptible 
ripple of muscular movement. This is the characteristic 
movement when the organism is not particularly stirred up. 
The crawling is the characteristic movement when the animal 
is, or has been recentl}', strongly stimulated. It is a purely 
muscular movement. 

a. Gliding. — The movement which I have called gliding 
is apparentl}'' the same as that which has been called " swim- 
ming " by Bardeen (loc. cit., p. 15), yet it must be stated that 
in all of my observations on a very large number of plana- 
rians I have never seen anything corresponding to some of 
the details which this author mentions in this movement. 
In the first place, he speaks of the worms moving progres- 
sively when not in contact with a solid body, i. e. of a 
movement freely through the water. This I am unable to 
understand, as I have never seen the slightest indication of 
the organism moving freel}'^ in the water without contact 
with a solid body or something which served the purpose of 



540 RAYMOND rEARL. 

a solid (viz. the surface film). The only possible exception 
to this is the passage of the animals from the surface film 
to the bottom on a string- of mucus, as described above. 
Furthermore, so far as I can find in the literature, no one 
else has ever seen a fresh- water triclad swim freely through 
the water. 

The movement takes place witli tlie body in contact with 
a surface either of a solid or of the surface film. There is, 
of course, between the ventral surface of the body and the 
surface on which it is moving, the thin layer of mucus which 
is constantly being secreted. Ifc is in this mucus layer 
rather than the free water that the cilia beat. 

This gliding movement is, so far as I have been able to 
ascertain, brought about by the action of the cilia on the 
ventral surface. There may also be some very slight 
muscular movement of the ventral body-wall comparable to 
that in the foot of a mollusc like Physa, which assists in the 
locomotion ; but in the case of the flat-worm this factor, if it 
exists at all, is vei*y insignificant. Only in a few instances 
have I been able to satisfy myself that any such movement 
was taking place, and then it did not have the characteristic 
rhythm seen in a mollusc. If this factor has any effect at all 
on the gliding movement ifc must be an extremely slight one. 

The cilia beat strongly backward, i, e. towards the poste- 
rior end of the bod3\ I have not been able to induce any 
reversal of the ciliary beats in these ventral cilia. Bardeen 
(loc. cit., p. 15) states that when the head is suddenly drawn 
back from some object the movement of the cilia on the 
antero-lateral margin of the head is reversed, and further 
suggests that " this i-eversed action may possibly be set up 
by the mechanical friction of the water." It would appear 
that the suggestion is the correct explanation, and that this 
is not a true reversal of effective beat. 

The cilia which are mainly effective in producing the 
gliding movement are distributed on the ventral surface of 
the body, as shown in Fig. 2. There is a band down the 
centre of the body, which widens out at the anterior end so 



MOVEMENTS, ETC., OF FRESH-WATKR TLAXAIUAN.^. 541 

as to cover nearly tlic Avliolc of the ventral surface of the 
head. The beat is the strongest down the median line of 
this band, and diminishes in intensity towards either edge 
until at the mai'gins there is no ciliary movement at all. At 
the anterior end the cilia near the side of the head beat 
backwards and at the same tiaie inward towards the median 
line, so that the currents take the course indicated by the 
arrows in that region in Fig. 2. The distribution and action 
of these cilia were made out by stirring finely powdered indigo 
in the water, and then either directly observing the ciliary 
action on these suspended particles as the animal glided on 
the surface film, or by indirectly observing it in a mirror 
placed below the bottom of the glass dish in which the 
worms were. Both of these methods gave the same results, 



^ 




Fig. 2. — Dingrani of the ventral surface of Plaiiaria, to sliow the 
distribution of cilia. The stijjpled area is that, which bears cilia. 
The arrows indicate the direction of the ciliary currents. (The 
pharynx is omitted for the sake of clearness.) 

and showed very clearly the distribution of the effective 
cilia. 

I have found no evidence of ciliary action on the dorsal 
surface of the body. Around the margins of the head there 
are cilia, but in other parts of the body, either on the dorsal 
surface or the edges, I have found no evidence of their 
presence. Particles of indigo dropped on the dorsal surface 
of a worm will remain in the same place for hours at a time. 
This is in striking contrast to the conditions in the land 
planariaus as described by Moseley ('77), Avhere the dorsal 
surface is thickly covered Avith cilia, which serve the purpose 
of keeping the body freed of foreign matter. 

In the gliding movement the head is raised slightly from 
the bottom so as to form an angle with the rest of the body. 

VOL. 46, PART 4. — NEW SERIES. N N 



542 RAYMOND PEARL. 

This position is shown in Fig. o. As will be brought out 
later, the head is a particularly sensitive portion of the bod}', 
and apparently its elevation is related to its sensory function, 
in that it practically brings the head into close relation with 
a large environmental field. The head is not held in a fixed 
raised position, but is in constant though slight movement 
whenever the animal as a whole is moving. These "feeling" 
movements ('Hastende Bewegungen ") of the head are very 
characteristic. The head as a whole is raised and lowered, 
and turned from side to side, while at the same time the 
antero-lateral margins are moved up and down and extended 
and retracted. These "feeling" movements of the head 
region are usually very slight, and escape notice except 
under close observation. When the organism is much 
stirred up, however, tliey may become quite apparent. 
Their purpose is evidently to increase the chances of re- 
ceiving stimuli, so that any stimulus in the neighbourhood 



Fig. 3. — Diuj^rammalic side view of a gliding plaiiaiiaii. 

may be quickly received. Constantly different sensory 
surfaces are presented to the environment. The head region 
acts in movement as a single great tentacle-like organ which 
is constantly testing the environment as the animal pro- 
ceeds. At the same time tlie auricles are fully extended and 
raised. I do not think that this marked sensory activity 
functions so much for the protection of the organism against 
harmful environmental influences as it does to give prompt 
notice of useful stimuli, — for example, stimuli due to the 
presence of food material. In general it would not appear 
that such an organism as the flat-worm runs as great i*isk of 
elimination from enemies as it does from not finding food 
material for its own support. In the ctenophore Mnemiop- 
sis Leidyi, whose reactions I have studioLl, no trace was 
observed of a reaction adapted to the purpose of getting the 
organism out of dangerous surroundings, but its only specific 



MOVEMENTS, ETC., OF FRRSH-AVATEIJ PLANARIANS. 543 

reaction is one which would bring it towards any food 
material which miglit be encountered.^ Wliile^ as will be 
shown later, there is in the case of Plauaria a reaction 
which is adapted to getting the organism, out of danger, yet 
it is not called forth by so weak a stimulus as is the food 
reaction, and it is evidently for the purpose of receiving 
stimuli of the lowest intensity that the "feeling" move- 
ments are adapted. 

In addition to the slight "feeling movements" of the 
head, described in the preceding paragraph, the organism 
frequently in the course of its gliding raises the whole 
anterior part of the body off the bottom and waves it about 
in the water. The portion of the body so raised may in- 
clude the whole anterior half. The gliding is usually 
entirely stopped or very much decreased in rate while these 
waving movements are taking place. The head is swept 
from one side to the other and raised high in the water, 
covering a considerable area. This movement is also un- 
doubtedly for sensory purposes. 

In the gliding movement the body back of the head is kept 
in an approximately straight line ; that is, there is no sinuous 
bending of the body such as is observed, for example, in 
Stichostemma (Child, loc. cit., p. 981), or at times in the 
movement of the earthworm. Furthermore, I have never 
observed any regular undulation of the margins of the body 
during movements such as take place in case of many polj- 
clads, e. g., Leptoplana tremellaris (cf. Lang, '84). 
Bardeen (loc. cit., p. 15) seems to imply that such motions 
occur, and are an aid in the locomotion, but I am unable to 
confirm this statement. There are, of course, slight move- 
ments and changes of contour of the margins of the body, 
but they are not of a prominence or character to warrant 
thinking that they in any way contribute to the propulsion 
of the animal. In fact, it seems more probable that they are 
in part passive results of the motion of the whole body, and 

' A brief preliminary account, of tlie reactions of Mncniiopsis has been 
publisiied in ' Science,' N. S., vol. xii, No. 311, pp. 927, 02S, : 00. 



544 RAYMOND PEARL. 

in part tlic expression of local changes in the tonic contrac- 
tion of the muscles. 

In the gliding movement the body is in close contact with 
the surface along which the animal is moving. When an 
animal passes from the resting condition into movement one 
can see the body lengthen and flatten so as to hug the 
surface. By observing with a compound microscope an 
animal gliding along the vertical side of a dish so that the 
edge is brought sharply into view, the closeness of the con- 
tact of the margin of the body with the surface can be well 
seen. Furthermoi'e, in specimens in which the posterior part 
of the body has been split longitudinally in the middle line 
to a point just behind the head, it is found that the half of 
the body which is determining the direction of the move- 
ment is always in close contact with the surface, while the 
other half only lightly touches it. 

It would appear from all the observations which have been 
given that the gliding movement is brought about in the 
following Avay : — The ventral surface of the body constantly 
secretes mucus in greater or less quantity. This mucus can 
be shown experimentally to be very sticky immediately after 
it is secreted into the water. As it is secreted under normal 
conditions it immediately sticks to the surface on which the 
animal is reposing. Thus there will be constantly between 
the animal and the surface on which it is moving a layer of 
mucus which is adherent to the substrate. We can think of 
the lowest part of this mucus layer where it is stuck to the 
surface as of denser consistency than its upper layers which 
are in contact with the animal. In this upper layer of the 
mucus the cilia are beating and constantly pushing the 
animal forward. Of course, what really takes place is that 
the cilia are pushing the secreted mucus backward, but as 
this layer of mucus becomes fixed to tiie substrate as soon as 
it is secreted, the practical result is that the animal is pushed 
ahead. 'IMiis relation is shown in Fig. 4. A represents a 
side view of a gliding worm ; D is the substrate ; C the cilia 
on the ventral side of the organism ; and 13 the mucus secre- 



MOVEMENTS, KTCI., OF FRESII-WATER PLANARIANS. 545 

tion, represented disproporfcioiiately exaggerated iu tliick- 
ness. This sticks to the surface of the substrate, and the 
backward beating of the cilia drives tlio worm ahead. 

]. Rate of Gliding Movement. — There is no very 
marked difference in the rate of the gliding movement in 
case of the species of l^lanaria studied. On the whole, 
specimens of P. dorotocephala move more rajjidly than do 
those of the other two species, but there arc large individual 
differences in this matter. Active specimens of Dendro- 
coelum, sp., move much faster than any other planarians I 
have observed. Large specimens of this form will sometimes 
glide along with simply amazing rapidity, not showing the 
slightest tremor of the surface of the body. 

As to the absolute rate of the crawling, some statistics 




yi^jy///f//Y///r7T//Tf/rr/r>J!:r//7Tt/^//-///:^/^7/,: 



Fig. 4. — Diagram Iu show \\\v. niecliauism of t.lic gliding movenient. A 
represents a specimen of I'laiiaria seen from t,lic side ; W, the layer 
of nniciis secreted by tiie animal. (This layer is represented as 
greatly exaggerated iu lliickness in |)roportioii lo the animal.) C, 
cilia. I), the subslrafe. The arrow indicates tJic direction in which 
the organism is moving. For further explanation see text. 

have been collected and will be presented. The statistics 
were obtained in the following way : — A paper was ruled into 
centimetre squares; over this was placed a Hat Petri dish 
containing the Avorm to be tested. Normal active specimens 
of P. maculata were used, and nothing was put into the 
dish but fresh clean water. The experiments were performed 
at night, and the source of illumination Avas a 16-candle 
power electric light enclosed within a ground glass globe. 
This lamp was 35 cm. above and 35 cm. distant iu a hori- 
zontal direction from the centre of the dish, so that the light 
struck the animal at an angle of approximately 45° on its 
dorsal surface. The worm was allowed to get into an even. 



546 RAYMOND PEARL. 

uoruial glitle^ and then to come around, as it usually would in 
a sliort time, so that it was headed in the direction of the 
light. Then the time which it took the worm to glide three 
centimetres was taken by means of a stop-watch. If the 
animal started crawling, or abruptly changed its direction, 
the trial Avas ruled out. The average rate in millimetres per 
second determined in this way from twenty trials on two 
individuals is 1'34. This rate is considerably higher than 
those obtaiued by I'arker and Burnett (: 00) for P. gono- 
cephala. In that form they found a rate of 1"04 mm, per 
second in the case of individuals moving toward a hori- 
zontal light; 1'12 mm. per second when movement was away 
from a horizontal light, and 1'08 mm. nev second when the 
animals were moving under a vertical light. There seems to 
be a well-marked correlation between the size of individual 
and the rate of gliding', as would be expected on general 
grounds, and is apparent from merely qualitative observations 
on the movement. One of the specimens from which obser- 
vations were taken was 11 mm. long when extended, and its 
rate of gliding was 1"48 mm. per second; while the other 
specimen, which was only G mm. long when extended, showed 
a correspondingly slower rate of l'2-3 mm. per second. The 
statistics are, of course, very meagre, and are not offered for 
any other purpose than to give a concrete idea of the approxi- 
mate rate of the gliding movement. A thorough quantitative 
study of this matter of the rate of movement in planarians 
and other related organisms, and of the effect of different 
agents on the rate, would, I believe, be very interesting, and 
might lead to valuable results. I hope to be able to make 
such a study at some future time. 

Lelinert (loc. cit., p. 17) gives a table of the rate of move- 
ment (presumably in the case of the fresh-water forms the 
rate of the gliding movement) of several species of flat-worms, 
lie gives no account of how the data were collected, but his 
values may be inserted here for the sake of comparison. His 
rates for Bipalium kewense and B. kewense viridis are — 
Usual rate, 1 to 1"33 mm. per second; occasional rate, 



MOVEMKNTS, ETC., OF FI{ESH-WATElt I'LANARIANS. 547 

l"8o mm. per second. Tliis agrees very closely with the rate 
for P. maculata given above (lo4). His rates for 
G e o d s m u s L i 1 i n e a t u s and D e u d r o c oo 1 u m 1 a c t e u ni are 
considerably slower (0"5 — O'GG mm. per second and 0"75 — ■ 
1*33 mm. per second respectively). Poly eel is tenuis 
.(1"66 — 1"83 mm. per second), Planaria polycliroa (2*16 — 
2"5 mm. per second, exceptionally 3*33 mm. per second), and 
Mesostomum tetragonum (2*GG mm. per second) show a 
markedly faster rate than the forms I have studied. 

Regarding the effect of different agencies on the rate of the 
gliding movement no special study lias been made, and I can 
only report a few incidental observations. Such a study 
should be made by exact quantitative methods, and this I 
have not had the opportunity to do. What the effect of light 
on the rate is, it seems to me, impossible to say with entire 
certainty. Cole and myself^ have found that light of great 
intensity (that obtained from a projection lantern with an 
electric arc as its source of illumination) causes a definite 
increase in the rate of gliding, but this increase has not been 
measured. The results oE Parker and Burnett do not help us 
to answer this question of the effect of the intensity of light 
on gliding, as they are concerned only with the direction of 
its rays. The well-known phenomenon of " Unterschieds- 
enipiindlichkeit " for light which Planaria shows (Loeb, 
'93, et al.) would indicate that increased light causes 
increased rapidity of movement. The electric current causes 
a very marked diminution in the rate of gliding in the weakest 
intensities which affect the organism at all. The effects of 
chemicals on the rate of gliding are not altogether uniform. 
Solutions of all chemicals tried with this point in view, when 
above a certain strength, caused a marked diminution in the 
rate of the gliding, or else an entire inhibition of it, and the 
substitution of some other form of movement. The action of 
weak solutions varied with the different substances. Very 
weak acids slightly increased the rate. AVeak sugar solutions 
had no observable effect so far as rate of movement was cou- 
' Unpubliahed observations. 



548 RAYMOND PEARL. 

cerned. Copper sulphate causes an entire inhibition of the 
gliding* movement in moderately weak solutions, even when 
these are not immediately fatal. Weak mechanical stimuli 
applied at the posterior end of the body cause a slight 
increase in the rate of gliding, but this is not marked, as any 
decided stimulus in this region of the body causes the crawl- 
ing motion to supervene. 

It is probable that the various agents affect the rate merely 
by causing changes in the general tonus of the animal. There 
is much evidence to support the view that the rate of gliding 
of an individual is a direct function of its tonic condition. 
Thus in the resting condition, which is characterised by a 
general lowering of the tonus, as will be brought out later, 
there is little, if any, ciliary movement. Again, after opera- 
tions Avhicli result in lowering the tonus, the gliding is very 
slow or entirely absent. 

2. Direction. — The direction of the gliding is, so far as 
my observations go, always forward. I have never been able 
to make the animal glide backward. This is in agreement 
with the finding of Child (: 01) in the case of St idle- 
st em ma. It, of course, indicates that the effective beat of 
the cilia cannot be reversed. In the case of the planarians 
on which this work was principally done, a lateral change 
of direction of movement is not brought about by the 
stronger beating of the cilia on one side. In other words, 
when the animal turns to one side it does so by a muscular 
bending of the body in that direction, and not by ciliary 
action. In an undetermined species of triclad, however, I 
found that the most usual method of turning towards one side 
was by the stronger beating of the cilia on the opposite side 
of the body. As an individual was gliding along the bottom 
of the aquarium dish it would swerve off at an angle to its 
former course without bending its body in the slightest 
observable degree. 

b. Crawling Movement. — The second form of loco- 
motor activity, the crawling movement, is distinctly a 
muscular movement. It takes place only when the animal 



MOVEMENTS, ETC., OF FUHSII-WATEK FLANAUIANS. 549 

Las beeu stimulated in certain ways, and is o£ much less 
frequent occurrence than tlic gliding. 

The crawling is always induced when the posterior end of 
the body is strongly stimulated. The characteristics of the 
movement are as follows : — If the posterior end of a worm 
which is gliding smoothly along is touched with a needle the 
posterior half of the body immediately contracts longi- 
tudinally ; an instant later the anterior end stretches out far 
in front and fastens to the substrate. Then there is a longi- 
tudinal contraction which begins just back of the head and 
runs posteriorly. This, of course, at once draws forward the 
posterior part of the body, and as this comes forward and 
gets a hold on the surface on which the animal is crawling, 
the anterior end is again extended far in front and attached 
to the substrate. This process is repeated until the animal 
settles down into the regular glide again. It consists essen- 
tially in a stretching out of the head followed by a pulling 
of the body forward by an active muscular contraction. 
When the animal is very strongly stimulated the por- 
tion of the body posterior to the pharynx usually takes no 
part in the crawling after the first general contraction. In 
fact, the posterior half of the body may even be held 
slightly raised off the bottom, while the region between the 
head and the origin of the pharynx is actively expanding and 
contracting and sending the body ahead. These strong ex- 
pansions and contractions of the anterior end which make up 
the crawling movement may follow each other in rapid 
succession as described above, or there may be a considerable 
interval between one contraction and the next. In this in- 
terval the body as a whole keeps moving ahead as a result of 
the ciliary action ; that is, the gliding movement continues 
during the crawling, so that the latter may be regarded as 
an additional movement for the purpose of advancing the 
animal faster than the gliding alone can do it. Tiie crawling 
may take place, however, with the ciliary beat entirely 
stopped. 

The duration of the crawling- movement after it is induced 



550 RAYMOND PEARL. 

is usually rather short. A single strong stimulus at the 
posterior end of the body — such, for example, as is given by 
running a needle through the body — will not usually cause 
more than three of the strong contractions of the crawling 
nioveraentj and then the animal will relapse into the usual 
glide. The limits in this matter I found to be from a single 
contraction and expansion as a minimum up to six or seven 
as a maximum. This is, o£ course, in response to a single 
stimulus only. By repeating the stimuli the animal may be 
made to continue the crawling indefiuitel3^ 

The effective rate of this form of progression is faster than 
that of the gliding movement. The crawling rate of one of 
the worms used for the measurement of the rate of gliding 
(the specimen 11 mm. long) was measured in the same way 
as was the gliding rate. The worm was stimulated Avith a 
needle at its posterior end just enough to keep it crawling, 
i. e. prevent it from settling into the regular glide. The 
average rate of the crawling was found to be I'GO mm. per 
second. Merely qualitative observation shows that the worm 
gets along somewhat faster in the crawling than in the 
gliding. 

1. Direction. — The crawling may take place so as to 
carry the animal either forward or backward. The back- 
ward crawling is induced by very strong stimulation of the 
anterior region of the body. It does not alwa3's occur tiven 
after such stimulation, there being apparently some in- 
dividual differences among the specimens in this respect. One 
factor which will call forth persistent backward crawling is 
partial desiccation. If the dorsal surface of the organism is 
allowed to dry, it will attempt to crawl backward violently. 
The mechanism of the backward crawling is jnst the reverse 
of that which obtains when the animal moves forward. The 
posterior end is extended and fastened to the bottom ; then 
a wave of contraction, starting in this case from the posterior 
end, draws the remainder of the animal backwards, and then 
the posterior end is again extended. The backward crawl- 
ing is usually induced when the worm is excessively stimu- 



MOVEMENTS, ETC., OF FliliSH-WATER J'L.\NAUIA?sS. 551 

lated or iujurcd at tliu anterior end. This moveineut almost 
always occurs when the head is cut off, aud may usually be 
induced in such decapitated specimens for a considerable 
period after the operation by stimulating the anterior end. 
The backward crawling* is not so rapid as the same move- 
ment forward. The reason for this appears to be that the 
posterior end is unable to take so firm a hold upon the 
bottom as does the anterior end. The backward crawling" is 
usually not very long continued, the animal soon coming to 
rest. The inability of the animal to glide in a backward 
direction should, of course, be noted in this connection. 
Strong chemical stimulation of the anterior end will cause 
the backward movement to appear in some cases. Ijight, so 
far as I have observed, will not, nor will the electric current. 
.There is considerable variation as to the appearance of this 
backward crawling. Some individuals cannot be induced to 
do it at all, or only in a very slight degree, while others will 
crawl backward for considerable distances after injury to the 
anterior end. It appears to be a complex of reflexes which 
under normal circumstances is inhibited, and only appears 
in any pronounced way under comparatively rare condi- 
tions. It is not, as might be expected, a method ordinarily 
used by the organism to get out of danger. This is one of 
the cases quite frequently met where an organism has among 
its available assets, so to speak, a reaction which is well 
adapted to a certain end, but of which use is not nuxde at all, 
or but very little. 

2. Stimuli which induce Crawling. — It may be said 
in general that almost any strong stimulus applied to the 
posterior portion of the organism causes the forward crawl- 
ing movement to ap])ear. Mechanical and strong chemical 
stimuli applied in this region will do this. Light, either of 
ordinary intensity, or of such high intensity as that from an 
arc light, so far as I have observed, will not cause the crawl- 
ing movement. The electric current does cause it, but 
greatly diminishes the rate. Any operative treatment — as, for 
example, cutting the body in two in the middle — almost in- 



552 RAYMOND PEAUL. 

variably causes the portion iu front of the cut to advance by 
the crawling movement, and, as has been mentioned in the 
preceding section, at the same time frequently causes the 
posterior piece to crawl backward. There is no reason to 
suppose that the operative procedure acts in this respect in 
any other way than merely as a strong mechanical stimulus 
applied at the point of the cut. Other stimuli which induce 
the backward crawling have been taken up in the preceding 
section. 

c. Movement on the Surface Film, — Motion on the 
surface film is practically confined to the gliding movement. 
This gliding is slower in rate than that on the bottom, largely 
on account of the greater flexibility of the surface on which 
the animal is moving. While the mechanism of the move- 
ment is the same in the two cases, the surface film is elastic, 
and does not give so firm a basis as does a solid body. The 
effect of this elasticity of the film is very well seen when the 
animal attempts to change its course and turn to one side. 
The film offers little resistance to the posterior end, so that 
this cannot easily serve as a fixed point for the anterior part 
to turn about. Furthermore, in case the anterior end is left 
in contact with the film when the turn is attempted, as is 
usually the case, there is almost as much resistance against this 
turning of the anterior part as there is resistance to hold the 
posterior end fixed as a pivot support. The consequence is 
that the worm is unable to change its direction of movement 
quickly when on the film, and it has to go through a succes- 
sion of muscular twists and jerks towards one side before the 
result is attained. I have not been able to induce well 
co-ordinated crawling movements in a worm while on the 
surface film. The preliminary contraction of the posterior 
part of the body occurs when that region is stimulated, but 
the subsequent stretching out of the head and drawing up of 
the body does not usually follow. I have tried stimulating 
both the exposed ventral surface of the animal and the dorsal 
surface from below, but neither method is effective. The 
reason for this is probably to be found again in the elasticity 



MOVEMENTS, ETC., OF FRKSII-WATEK TLANARIANS. 553 

of tlie film. The iiiiterior end is unable to get any firm 
attaclunenb so tliat tlie rest of the body may be drawn 
forward. Furthermore, similar resistance is offered to the 
stretching- of the head forward as to the turning of it 
towards one side. 

When the animal is gliding on the surface film the same 
raising of the head (with reference to the worm, of course: 
in this case, a lowering with reference to the centre of the 
earth) and waving it about in the water occurs as under 
normal circumstances. In some cases two thirds or three 
fourths of the whole body will be thus raised and waved 
about, ex;tending itself to its utmost capacity, and apparently 
seeking some solid body on which to attach itself. In these 
cases only a small portion of the very posterior end of the 
body will be left in contact Avith the film to support the 
whole. 

On coming to the side of the dish when gliding on the 
surface film, the worm almost invariably leaves the film and 
turns down the side of the dish. The reaction which is the 
cause of this and of the organisms passing from the side of 
the dish on to the film will be brought out later. 

d. Relation of Movements of Triclads to those of 
other Forms. — In respect to their movements, the triclads 
studied occupy a somewhat intermediate position between 
certain othei* groups. The rhabdocccles are in general 
characterised by free movements in the water, brought 
about by cilia covering the whole body. Their movement 
in general features resembles that of the liolotrichous 
Infusoria. A type showing well this class of movement 
among the Turbellaria is Stenostoma leucops. The 
movement is not at all or very little dependent upon muscular 
activity. On the other hand, the movement of many of the 
polyclads is characteristically muscular. An example of this 
is found in the case of Leptoplana tremellaris, where 
the movement is largely effected by the rhythmical beating 
of the margins (cf. Lang, '84). In fact, this form of 
movement has become so well developed in these animals 



554 RAYMOND PEARL. 

that we may think of tlie margins of the body as special' 
locomotor organs. Ciliary action plays little if any part in 
the movement of sucli a form. It is to be noted, however, 
that both the rhabdocoeles and the polyclads are capable of 
performing true swimming movements, i.e. movements free 
in the Avater withont contact witli any solid body. In the 
fresh-water triclads, especially of the genera Plannria 
and Dendrocoelum, the cilia have become much diminished 
in comparison with the rhabdocoeles, and are restricted to a 
portion of the ventral surface only. Consequently they are 
not numerous and strong enough to support and move the 
disproportionately heavier body freely through the water. 
The movement of the cilia merely serves in these forms to 
propel the body while insufficient to support its weight. 
Consequently we find the principal form of movement to be 
a gliding over the surfaces of solid bodies.^ On the. other 
hand, the fresh-water triclads have not attained the liigh 
development of muscular locomotion which the polyclads 
have. There is a purely muscular movement in their case, 
but it is not by far the most important form of locomotion, 
and is not so highly developed as is that of the polyclads. 
Evidently, then, the fresh-water ti-iclads seem to form a 
transitional stage in respect to locomotor phenomena between 
the rhabdocoeles on the one hand, where purely ciliary 
locomotion obtains, and the polyclads on the other hand, 
where we find the locomotion largely if not entirely 
muscular. Whether this has any phylogenetic significance 
is not certain. 

The land planarians occup)^ a position very similar to that 
of the fresh-water forms so far as their movements are 

' I do not wisli to imply, in tills discussion of tlie different forms of move- 
ment as related to the number and distribution of tiie cilia, any belief that 
structure gave rise to function or function to structure. I wish merely to 
point out the evident correlation wiiich exists in the matter. It seems to me 
most probable that structure and function changed together ; but in this, as 
in many otlier similar cases, positive evidence is lacking, and consequently 
attempts to settle the phylogenetic development of the plienomena would 
appear to be fruitless. 



MOVEMRNTS, KTC, OP FRESH-WATER PI-ANARTANS. 555 

concerned. There is, however, a notewortliy difference 
between the two groups. In tlie movements of tlie hind 
planarians the muscular factors (rhythmical wave motion of 
the ventral surface, and. snake-like movements of the whole 
body) are more important relatively to the ciliary component 
than in the fresh- water forms. In these land planarians 
there is evidently the beginning- of the characteristic 
rhythmical wave motion of the part of the body in contact 
with the substrate, Avhich reaches its highest development in 
the case of the Molhiscn. 

II. Non- Loco motor Movements.^ 

Under non-locomotor movements will be included the 
phenomena of contraction, expansion, " feeling movements," 
movements of the pharynx, etc. The purpose of discussing 
these phenomena, which are not immediately included in the 
general standpoint, is to give an account of their mechanism 
which may be referred to in succeeding portions of the paper. 
These movements are the physiological foundations on which 
the locomotor movements and the reactions are based, and it 
is necessary to determine their mechanism in order to bring 
the analysis of the behaviour to completion. 

a. Contraction of the Body. — By the term '^contrac- 
tion of the body," when applied to forms like the flat-worm, 
is usually meant the sliortening of the body lengthwise. In 
the flat-worm this movement is brouglit about by tlie con- 
traction of the longitudinal muscle-fibres. It ma}' involve 
the whole body or only a portion of it. Most frequently only 
a part of the body contracts longitudinally after stimulation ; 
thus, if the anterior end is rather strongly stimulated in the 
middle line, the resulting contraction will usually involve only 
the anterior third of the body. In this longitudinal contrac- 

' III discussing I lie nius-culaluic I liave usetl tlirouiiliout, tlie iiomcnciature 
of Jijima (loc. cit.), in wliose paper a very full (iescriptiou of this system will 
he fouiul. I have identified in sections of V. niaculata the following groups 
of muscle-fibres: — («) outer longitudinal, (il circular, (c) oblique (?), (d) inner 
loiiL;itudinal, (f) dorso-veiilral, (/) transverse. 



556 EAYMOXI> PEARL. 

tion all the sets of muscle-fibres other than the longitudinal 
must be relaxed completely, because as the animal shortens 
it grows broader and thicker, which would be impossible if 
the ring, or transverse or dorso-ventral musculature, also con- 
tracted. The longitudinal musculature is apparently better 
developed and more effective on the ventral side of the body 
than on the dorsal, because after very strong stimulation of 
the anterior end there is a well-marked tendency for the 
middle portion of the body to be raised and the head some- 
what curled in under it. This relation is shown diaoram- 
matically in Fig. 5. This curliug under of the head does not 
appear to be a specific reaction, but, on the contrary, merely 
an expression of the fact that the ventral musculature is 
capable of shortening its side of the body more in maximal 
contraction than the dorsal side. Jijima (loc. cit., p. 378) 




Fig. 5. — Diagram showiiif^ the appearance in side view of a 
niaxinialiy contracted planarian. 

finds, from a histological study of the musculature, that the 
bundles of fibres in the main longitudinal muscle layer are 
thicker on the ventral than on the dorsal side. 

b. Extension of the Body. — The mechanisms by which 
extension of a soft-bodied animal is brought about are 
probably very different in different groups. In the case of 
the flat-worm extension is produced by the contraction of the 
circular muscular layers surrounding the body, and of the 
transverse and dorso-ventral systems of musculature. Probably 
also the oblique musculature, when present, assists, by its 
contraction, in the extension of the body. The mechanical 
necessity for extension of the bodj^, after contraction of these 
muscles, is readily apparent. If the body, for simplicity's 
sake, be considered a cylinder, contraction of circular 



MOVEMENTS, ETC., OF FRESH-WATER PLAXARIAXS. 557 

muscles must cause it to lengthen, wliile witli the form of 
body which really exists in the flat-worm, the contraction of 
the well-developed dorso-ventral fibres must bring about the 
flattening seen in the fully extended gliding auimal. 

Probably by far the most important sets of muscle-fibres 
for producing the general extension are the dorso-ventral and 
circular. It is to be noted that contractions of any of the 
sets of fibres may take place in localised regions, producing 
extensions or contractions of that region, according to the set 
affected. The sensory, or " feeling " movements of the head 
are brought about in this way. 

The extension, and probably also in large part the extrusion 
of the pharynx is brought about by contraction of its well- 
developed circular musculature. 

In the case of the marine mollusc, A ply si a li niacin a, 
Jordan (: 01, pp. 11 — 15) has recently shown that extension is 
brought about in an entirely different manner. It results 
from passage of fluid from vesicles in the skin into the spaces 
in the body parenchyma when the body muscles are relaxed. 
When the animal conti^acts this fluid (blood) is pressed out 
into the vesicles, which become very much extended and 
swollen ; then, when the muscles are relaxed, the elasticity of 
the walls of the vesicles forces the fluid back into the body, 
and thus causes its extension. As a result of this method of 
expansion it is possible to kill the animal fully extended by 
the use of such poisons as cocaine. 

It is not unusual to consider the fully extended condition 
of such an organism as a flat-worm as one of approximate 
relaxation. Instead of this, it is, in fact, a condition in which 
a certain part of the musculature is in a state of well-marked 
tonic contraction. This furnishes a reason for the fact that 
it is impossible to kill these animals in a completely extended 
condition by the use of poisons which tend to produce a 
relaxation of the muscles. Under these circumstances the 
animals take on the typical relaxed form, which is quite 
different from that of extension. 

c. Rest. — Inasmuch as a very large, if not the larger 

VOL. 46, PART 4. NEW SERIES. O 



558 RAYMOND PEAKL. 

portion of the life of a planavian is spent in a condition of 
rest, it will be well to discuss tliis matter ; and it may, per- 
haps, best be taken up under the general heading of 
''activities," although really the opposite of activity. 

The appearance of the worm when resting is, as has already 
been mentioned, quite different from its appearance in the 
active condition. The body is shorter, wider, and thicker. 
The ordinary contour of the head is almost entirely lost, and 
in place of the sharply pointed anterior end of a form like P. 
dorotocephala, the end is evenly rounded. The auricles 
disappear almost entirely, and their position is indicated only 
by the difference in the pigmentation at that part of the 
dorsal surface. The lateral edges of the body frequently have 




j^iG. G.— Diagram sliowing the typical appearance of a resting planarian. 
Tlie dolAed line bounds approximately tlie area covered in tlie 
" testing movements " wliicii precede the coming to rest (cf. text). 

a wavy line instead of the straight one of the active condi- 
tion. The anterior end of the body is in contact with the 
bottom, and not raised as in movement. The general appear- 
ance of a resting planarian is shosvn in Fig. G. 

The coming to rest of a gliding animal is usuall}' done in a 
very characteristic way. First, the animal glides more and 
more slowly for some distance before reaching the point at 
which it will finally stop. The distance before reaching the 
stopping place in which the worm glides appreciably slower 
is not however, in most cases very considerable — usually not 
more than two or three times its own length. It is to be 
noted that this slower gliding which precedes the coming to 



MOVEMENTS, ETC., OP PRESH-WATKR PLANARIANS. 559 

rest is not in form or rate distinguishable from the other 
slow gliding motion of the worm which is not followed by 
rest. In other words, a specimen may glide slowly for a long 
time without stopping, so that one cannot prophesy with 
certainty from the rate of movement whether the specimen is 
soon coming to rest or not. The coming to rest is practically 
always preceded by a period of slower gliding, but all slow 
gliding is not immediately followed by rest. After a brief 
period of this slower gliding the worm suddenly stops, and 
the posterior half of the body remains fixed in precisely the 
same position. The anterior half of the body is slowly moved 
about over the bottom from side to side, the head being 
touched frequently to the bottom or any other solid object in 
the neighbourhood. The anterior part in this "feeling" 
movement moves about the posterior part as a fixed point, 
the latter very rarely changing its position after it has once 
stopped. The thoroughness of this "testing" of the sur- 
roundings by the sensory anterior end varies much in different 
cases, but in practically all cases one can see some indication 
of it. I have in some instances seen it done very thoroughly, 
so that the whole surroundings within a radius of 3 mm. 
were gone over. Finally, when this is done the animal comes 
to complete rest, and assumes the typical relaxed condition 
shown in Fig. 6. The apparent significance of the " testing " 
movements at the time of stopping is that it is apiece of pro- 
tective behaviour. The worm examines the surroundings 
before coming to rest, to see if there is anything dangerous 
(either of a solid nature or a harmful chemical) in the 
immediate neighbourhood. Whether or not this explanation 
is the true one, and further, whether natural selection 
developed this reaction for protective purposes, seems to me 
to be very doubtful, for reasons brought out in anotlier place 
(cf. pp. 542 and 543). In some cases I have seen the Avorms 
come to rest by simply stopping without any appreciable trace 
of the " feeling " movements, but this is not the usual pro- 
cedure. In coming to rest in one of the collections already men- 
tioned, the '^feeling" movements are usually very well marked. 



560 



RAYMOND PEARL. 



There is a "well-marked tendency for the planarians studied 
to come to rest in such a way that the long* axis of the body 
forms a right angle, or nearly a right angle, with the lines 
of the force of gravitation. The cases in which the organisms 
come to rest with the long axis forming an angle of less than 
thirty degrees with the line of gravitation are rather few. 
Of course, when they come to rest on the bottom, the angle 
formed is approximately ninety degrees. A large number of 
observations on individuals which came to rest on the sides 
of dishes of various shapes have given the general result 
stated above. There are, of course, exceptions, but there is, 




Fig. 7- — Diagram showing (lie positions taken by planarians coming to 
rest iu a dish (see text). 

after making due allowance for these, a tendency to a hori- 
zontal position as the position of rest. This is the only 
behaviour of the organism which bears any resemblance to 
a geotactic reaction. Another tendency, less marked than 
the former, is for the animals to come to rest in the angle 
formed by the sides and bottom of the dish. Not only do 
specimens come to rest lying directly in the angle, as shown 
at A in fig. 7, but also, and more frequently, they lie in such 
a position that a part of the body is on the side of the dish 
and a part on the bottom, as shown at B, fig. 7. In this 
position the anininl usuall}' lios obliquely rather than at right 



MOVEMENTS, ETC., OF FIil!]SH-WATEli PLANARIANS. 5G1 

angles to the line of the junction of the side and bottom of 
tlio dish. The animals usually come to vest in this position 
after they have been gliding on the side of the dish. When 
they come to rest from movement on the bottom of the dish, 
a position frequently taken is that shown in C, fig. 7, where 
only the very anterior end is in contact with the side. This 
coming to rest in the angle of a dish is apparently a reaction 
which agrees with those usually called thigmotactic reactions. 
But it is not, as has been stated by several writers, due to a 
tendency to get more of the body in contact with something 
solid^ than is in such contact under usual conditions; for in 
the case of an organism like a flatworm, it is impossible for 
any more of the surface of the body to be in contact with a 
solid when it is bent, as shown in Fig. 8, A, than when it is 
flat, as in B. There is the same amount of surface in contact 
in either case. The ventral surface of the flat-worm is 



L 



A B 

Fig. 8. — Diagrammatic cross-sect ion of a ])laiiaiiaii at rest — A, in 
an angle ; and B, on a i)!ane surface. 

strongly positively thigmotactic under all circumstances, and 
the dorsal surface negatively thigmotactic, but this does not 
help us understand why the animal comes to rest frequently 
in angles. This behaviour of the flat-worm in dishes is due to 
the same sort of reaction as that which causes them to come 
to rest on unevennosses on rocks, and also causes the same 
phenomenon in a more marked degree in the case of Lit- 
torina, as recently described by Mitsukuri (:01). The 
common factor in the reaction in all cases is that difl^erent 
parts of the body are brought into such positions that they 
form unusual angles with each other. Since this phenomenon 
is distinctly different from any embraced by the term thig- 
motaxis as used in its true sense, it seems desirable that it be 



562 EAYMOND PEARL. 

given a specific name. I would propose for this reaction the 
term gouiotaxis.^ 

When a flat-worm starts from a resting condition tlie 
nature of the movement, i. e. whether gliding or crawling, 
depends in large measure on the intensity of the stimulus 
which starts it. If the resting animal is rather strongly 
stimulated it will start at once into a crawling movement, 
which changes to gliding after three or four, or fewer, con- 
tractions, ])rovided the stimulus is not again renewed. It is 
possible by the use of a very weak stimulus to start the 
resting animal off at once into the gliding movement, or with 
only the faintest indication of a single crawling contraction. 
When the animal starts spontaneously into movement it 
usually begins at oace with the glide. When starting spon- 
taneously the glide is usually preceded by some of the 
" feeling " movements of the head end, such as precede the 
coming to rest. The purpose of these is evidently the same 
as in the former case. Any sort of strong stimulus will start 
the resting animal into movement. 

The physiological condition of the resting animal is, as has 
already been mentioned, one of relaxation. All of the 
muscuUir systems of the body are in an apparently com- 
pletely relaxed condition. This is evidenced by the form of 
the resting animal, which differs from that of one in move- 
ment, in being shorter, wider, and thicker, and in not showing 
such features as the auricles, or the pointed tip of the head. 
This relaxed condition is evidently one of lowered tonus, as 
may be determined by simple observation. Stimuli of an 
intensity which would cause a marked reaction in an in- 
dividual in an active condition, will produce no effect on a 
resting animal. This point has been tested with a variety of 
stimuli, including- mechanical, chemical, food, etc., and the 
markedly loAver tonus of the resting animal is very evident. 
The reactions which are produced, provided the stimulus is 
made strong enough to be just effective, are weak. Of 
course, if the stimulus is above a certain strength, or is con- 

' From ywri'a = angle. 



MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 563 

tinned for some tiinOj tlic ciuiinal will b(!couiu generally 
stirred up and glide or erawl away. This condition of 
lowered tonus in the resting animal reminds one of the con- 
ditions found in sleep in the higher animals. There, as in 
this case, the general sensory and muscular tonus is greatly 
reduced, and there seems to be no good reason why the 
resting condition of these lo.wer organisms may not be con- 
sidered and called '' sleep." The two things appear to be 
fundamentally the same physiologically, and would appear 
to serve the same purpose. Furthermore, there is no 
apparent reason why the lower organisms should nut have as 
great a need as the higher for periods of rest or sleep, 
during which the anabolic processes are in considerable 
excess over the katabolic. The fact that some lower 
organisms are so balanced physiologically that they 
apparently do not require such periods of rest is not con- 
clusive evidence that other low organisms must be similarly 
balanced. So far as is known to the writer, there has been 
comparatively little attention paid to the physiological con- 
dition of lower organisms during different phases of their 
activities. An animal which is not moving is loosely said 
to be in a " resting condition," Avhen in nuiny instances, 
as in Clepsine, the quiet animal is in a condition of 
heightened rather than lowered tonus (cf. Whitman, loc. 
cit.). 

As was noted ia the section on " Natural History," the 
periods of activity of PI an aria are separated by periods of 
rest of greater or less length. The time spent in the rest- 
ing condition, at least during the daytime, is considerably 
greater, on the average, than that spent in movement. 
Probably, however, this is reversed during the night, when 
the activity is greater than during the day. This periodicity 
in the activity is just what would be expected if there is a 
necessity for rest at intervals as in the higher animals. 

The causes which immediately induce the coming to rest 
may now be considered. The principal cause, as has been 
indicated above, is that the animal becomes fatigued by 



564 RAYMOND PEARL. 

movement, and its general tonus becomes lower and lower. 
As a result of this it must remain relaxed for a certain time 
in order that recovery may take place. When in the course 
of the activity of the animal its general tonus gets below a 
certain point it stops, the actual process of coming to rest 
being a more or less gradual one. A strong piece of 
evidence in favour of this view is the fact already given in 
the section on " Natural History/' namely, that if the 
animal is stirred up and made to start moving again im- 
mediately after coming to rest each time, it will be found 
that the periods of activity become progressively shorter. 
Furthermore, when the general physiological condition of 
the organisms is weakened by keeping them for a time in 
the laboratory, it is found that the periods of rest become 
progressively longer in proportion to the periods of activity. 
The general " predisposing condition " to the coming to rest 
is then probably a lower tonus due to fatigue. The im- 
mediate causes determining the exact place chosen are of 
three sorts. First, and probably most important of these, is 
the intensity of the light. It is well known that planarians 
tend to come to rest in regions of comparatively low light 
intensity, the reaction having been first noted by Loeb ('93), 
and called by him " Unterschiedsempfiudlichkeit." This 
factor seems to bo the most important of any in determining 
the region in which the animals come to rest, both under 
experimental conditions and in the natural habitat. In 
aquarium dishes placed close to a window, and containing 
considerable plant material, the worms will be found resting 
practically always in the half of the dish away from the 
window. The largest number of individuals will be 
entangled in the plant material, and usually for the most 
part invisible ; while of those specimens resting on the sides 
and bottom of the dish the greatest number will be found in 
such places that there are heavy masses of plant material 
between them and the window. A few will come to rest far 
around on the sides of the dish where the glass itself cuts off 
some of the light. This last position has been mentioned by 



MOVEMENTS, ETC., OF FUESH-WATER PLANARIANS. 565 

Loeb as the oue most ffequently taken by plauarians in a 
dish containing' only water. Tiiis behaviour towards light 
is not, however, an absolutely precise reaction. Many times 
during- experiments I have seen specimens come to rest in 
the vei-y lightest parts of the dish and remain there ; but in 
general this reaction will cause most of the animals to gather 
in shaded areas. It is probably the principal factor in 
causing the animals to take positions beneath stones in their 
natural habitat. 

The second immediate cause in determiuiug where the 
animals shall come to rest is the goniotaxis mentioned above. 
If an animal in the proper physiological condition of reduced 
tonus comes to an unevenness in the surface on which it is 
moving, it will in most cases come to rest there. This, again, 
is not a very precise reaction ; not sufficiently so as to make 
it possible to predict beforehand where any given individual 
will stop. In this case, just as in the case of light, much 
depends on the animal's physiological condition, and when in 
the proper condition they may come to rest on a perfectly 
smooth surface. Thus in a dish individuals will always be 
found at rest on the smooth sides and bottom, yet there is a 
distinctly marked tendency, when the animals are put under 
experimental conditions and closely observed, for them to 
come to rest in the angle of the dish. This reaction probably 
also plays a considerable part in the habit of coming' to rest 
among the branches and leaves of the plant material. In 
the natural habitat it is undoubtedly the factor which causes 
them to take positions on the uneven parts of stones. It 
may be that the immediate cause of the stopping in this case 
is the increased i-esi stance to movement afforded by the 
unevenness of the surface. This, acting on an animal in a 
fatigued condition, might give the necessary stimulus fur 
the stopping. 

The third factor in doterminiug where the animals shall 
come to rest is one about which I am doubtful. There seems 
to be some evidence, from the behavioui" of the animals 
themselves, that in the foimatiou of the groups or collections 



566 RAYMOND PEAUL. 

previously mentioned, (pjj. 533, 534) tliere is a sort of cliemo- 
kinosis. That is to say, the presence of some chemical 
substance in the water causes the animals to stop. The 
evidence for this factor will be taken up with the discussion 
of the formation of collections. It probably does not play 
any part in determining where a single individual shall come 
to rest outside of a collection. 

It must be emphasised that all of these three factors are 
secondary in importance as compared with the physiological 
condition of the animal, which may be said to prepare it for 
the resting state. An active animal, in which the tonus is at 
or near the maximum, will pass through regions of low 
illumination, uneven surface, or collections of other indi- 
viduals without stopping. Only when the animal is in the 
right general condition do these factors come in to determine 
the precise point where the stop shall be made. 

1. Formation of Collections. — Since the formation of 
collections is dependent on the animals coming to rest in a 
certain area, it ma,y properly be taken up in this section. 
The collections are fairly well-defined groups of from six or 
eight up to twenty or more individuals. The general appear- 
ance of such a gi'oup is shown in Fig. 1. The individuals 
composing it have no definite orientation, but are scattered 
about with the anterior ends directed in whatever way they 
happened to be pointed when the individuals stopped. The 
distance separating the individuals varies much in different 
cases. In some cases it may be as much as a half-centimetre, 
or again may be the Avidth of an individual worm or less. 
This formation of collections of this sort might be considered 
the result of a " social instinct " by animal psychologists of 
the Binet school. Actually, it appears to be due to two 
simple reactions taken in conjunction with the general 
physiological condition of the individuals composing it. The 
first of these reactions is that to light. That is to say, when 
individuals come to a comparatively restricted area of a 
certain degree of illumination, if they are in a certain 
condition of reduced tonus, they stop. Those which are 



MOVEMENTS, liTC, OF FRESH-WATER FLANARIANS. 567 

very active pass on through the region, bnt necessarily in 
course of some time several individuals will have stopped, 
and a group will have been begun. Wlieu once started 
another reaction apparently enters to assist in enlarging it. 
This reaction appears to be due to some chemical substance, 
and belongs to the class of reactions which Eiio-elmann has 
suggested should be called "kinetic,'^ in tliis case chemo- 
kinesis. It would appear that planarians excrete or secrete 
some cliemical substance towards which they are themselves 
positively chemotactic, and which also causes them to come 
to rest. When several individuals remain quiet in a small 
area this substance, of course, accumulates and affects other 
individuals passing. That some such a substance is sepa- 
rated from the bodies of the animals is evidenced by two 
phenomena. First, in the case of the food reaction, which 
will be taken up in detail later, it is found that after one or 
two individuals have attached themselves to a piece of food 
material and begun feeding the mass of food and planarians 
is a much more effective stimulus to positive chemotaxis than 
is the same food substance alone, even though it may have 
remained in the water a greater length of time. The "zone 
of iufluence " (vide infra, p. 626) of the food and feeding 
individuals together is much wider than that of the food 
alone. Specimens are affected at a greater distance from 
the food and react more sharply. As a result of this, dense 
aggregations of planarians will be formed in a comparatively 
short time after the first two or three individuals have found 
a bit of food. As there is no reason to suppose that the 
action of the food itself is different in the two cases, we 
must conclude that the greater effectiveness of the food and 
feeding indivitluals is duo to some chemical substance coming 
from the organisms themselves. 

The second line of evidence for the existence of a reaction 
to a chemical in the formation of collections is found in the 
behaviour of specimens coming near a group of individuals 
resting on the bottom of a dish. When some distance away 
from the outer boundary of such a group a gliding animal 



568 RAYMOND PEARL. 

will frequently be seen to give a well-defined positive 
reaction, and turn towards the group. The reaction is of 
precisely the same character as that given by the organism 
to weak chemicals (to be described later), and the behaviour 
convinces one observing it that the specimen is stimulated 
by some chemical diffusing out from the group. After 
turning towards the group the specimen will glide into it 
and usually coiiie to rest, in the manner which has been 
described above. 

What the nature of the chemical substance present in the 
region about the groups is, I have not been able to discover. 
Neither rosolic acid nor methyl orange is discoloured by it. 
Whatever its nature, it must be in an extremely diluted 
state. This seems evident for two reasons : first, because it 
does not affect delicate indicators ; and second, because it 
does not have any effect on active specimens of Plan aria. 
A large number of experiments have been performed to test 
this latter point, but always with the same result. Unless 
the individuals were in the proper predisposing condition of 
lowered tonus, they would pass by or through groups of 
other individuals without giving any reaction. 

Attempts to produce, artificially, collections of planarians 
in chemicals have been unsuccessful. I have tried various 
solutions (such as sugar, weak alkalies, etc.) to which the 
organisms showed a well marked positive chemotaxis when 
tested by other methods, but have not been able to get any 
formation of collections in them. The animals would give 
the positive reaction on coming to the edge of the diffusing 
chemical and pass into it, but would not come to rest. This 
failure to produce collections artificially is not surprising 
when one considers the number of conditions necessary for 
the production of the desired result. The organism must be 
in just the right physiological condition, the chemical must 
be of a certain concentration, and finally, it must be located 
in an area of a certain light intensity. It is practically 
almost or quite impossible to fulfil all these conditions at 
the same time in an experiment. 



MOVEMENTS, ETC, OP FRESH-WATER PLANARTANS. 569 

The coniing- to rest in the collection seems to be due 
simply to the direct effect of the chemical on the org-anism. 
'J'here is no evidence that the animals are held in the group 
as a result of a negative reaction to the surrounding water, 
as is the case in the collections formed by the infusoria 
(cf. Jennings, '99, h). The method of formation of collections 
in chemicals in the case of the infusoria is as follows : — 
Specimens swimming about at random come to the edges of 
drops of chemicals purely by chance. If, for example, the 
chemical happens to be a weak acid, the specimens will pass 
into the drop without giving any reaction. When, however, 
they reach the opposite edge of the drop and attempt to pass 
from the chemical back into the water they are stimulated, 
and give their usual motor reaction. This turns them back 
into the drop, in which they are, as it Avere, '' caught in a 
trap." As a consequence of this method of reaction a very 
dense collection will be formed in a shoit time. With the 
flat-worm the case is very different in that an active in- 
dividual frequently passes into and out of one of these 
collections without showing the faintest trace of a reaction 
on either side. The only way in which any stopping in the 
region is brought about by a chemical is by a chemokinetic 
reaction. The fundamental difference in the reactions of the 
two groups of organisms on which this difference in the matter 
of forming collections is based will be brought out in the 
section on the reactions to chemicals. 

To sum up, the formation of collections of individuals 
seems to be due, in the first instance, to the tendency of the 
organisms to come to rest in areas of a certain degree of 
intensity of light, and in a lesser degree to a tendency to 
turn towards and come to rest in areas containing some 
substance secreted or excreted by the worms themselves. A 
prerequisite in the formation of collection, as in the coming 
to rest under any circumstances, is a proper physiological 
condition of reduced tonus. 

There does not appear to bo any special biological sig- 
nificance to this tendency of the animals to collect in gi'oups. 



570 



RAYMOND PEARL. 



The behavionr is not of auy evident benefit to the organisms, 
as it is in the case of infusoria, where it is apparently closely 
connected with the obtaining of food. On the contraiy, it 
seems to be, at least potentially, a harmful thing, because 
any accident or enemy would affect a number of individuals 
rather than a single one when they were so collected. 

d. The Effect of Operations on Movement. — It 
may be well to put together in one place the results which 
were obtained with reference to the movements from animals 
which had been cut in various ways. From these we can 
form some idea of the relation of the nervous system of the 
planarian to its movements.^ 

The immediate effect of any operation is that of a very 
strong mechanical stimulus applied to the same part of the 




Fig. 9. — Operation diaj^rani. Tlie heavy str.night, line indicates tlie cut 
made. For results see text. 

body, and the sort of movement resulting in each piece 
depends on the position of the cut. The details of this im- 
mediate effect will be described in connection with other 
mechanical stimuli. What concerns us here is the permanent 
after-effect of operations on the movements. We can best 
get at this matter by taking up some specific cases. 



^ All the operations were performed with a sharp scalpel, in most cases 
with the specimen in a dish of water. In some cases the worm was transferred 
to a drop of water on a soft board for the cuttins', but in all cases where 
immediate observations were wanted, the operations were performed in tlie 
dishes used for t,lie experiments. The only difficulty in performing operations 
on planarians arises from tiie fact that if the edge of the knife is allowed to 
rest on the surface of the body for even a very short time before the cut is 
made, it will become covered with the sticky slime from the aninuil, and tiien 
any clean cut is impossible. The edge will slip off the back of the worm 
without iicnct rating. 



MOVKMENTS, ETC., OF FRESH-WATRR TLANArjAN?. 571 

If a planai-ian is cut scjiiai-ely across the body in tlie 
I'egion a short distance behind the head, as indicated in 
Fig. 9, the anterior piece will continue to move after tlie 
operation at approximately the same rate as the whole 
animal did before. After the immediate effect of the opera- 
tion is past the glide is its ordinary movement, and it will go 
al)out the dish and behave in general like a whole individual. 
At the outstart its pei-iods of activity and rest are distributed 
about as in a normal individual, or, in other words, its power 
of spontaneous movement is not inipnired, at least for a time. 
On the other hand, the posterior piece comparatively soon 
comes to rest after the operation. Its gliding movement is 
slower, and the periods of rest become longer and longer in 
compai'ison with the periods of activity. Its power of spon- 
taneous movement becomes very greatly diminished within a 
comparatively short time after the operation, and it remains 



Fig. 10. — Diagrammatic side view of a decapitated specimen performing 
the gliding movement. 

in the relaxed resting condition during the greater pnrt of 
the time spent in the process of regeneration. When this 
posterior piece does glide about soon after the operation its 
anterior end is usually raised off the bottom considerabl}- 
higher than is the head of a normal flat- worm under similar 
circumstances. This is shown in Fig. 10. There are no 
''feeling" movements of the anterior end of such a piece, 
but instead this end is held very stiffly in the raised position. 

If, instead of making the cut so close behind the head, it is 
made back in the middle region of the body, the anterior 
piece behaves as before, i. e. like the normal animal. The 
posterior piece, however, moves slower thau did the corre- 
sponding piece in the previous experiment, and it comes to 
rest sooner after the operation, and remains quiet longer. 

In the same way cuts may be made nearer and nearer the 
posterior end ; the posterior piece will move more and more 



572 RAYMOND PEAEL. 

slowly, and come to rest sooner. At the same time the 
anterior pieces will appear more and more like the normal. 
In both sets of pieces the crawling movement may be in- 
duced by proper stimulation of the posterior ends. 

01)lique transverse cuts produce the same results as do 

A 



B 




Fig. 11.— Operation diagram. Heavy lines indicate the cuts made. 
For results see text. 

direct ones. The same laws hold as to the movements of the 
pieces. In case a strip is cut from the side of the body, as 
shown in Fig. 11, the smaller piece A curls np, and does not 
make any further progressive movements, although it 
remains alive, and will eventually regenerate in most cases. 
The main part B contracts on the cut side, and hence becomes 
curved in that direction after the operation. It is able to 
move about, but at a somewhat slower gliding rate than 
normal, and in a path curved towards the cut side. In case 
a worm is slit down the middle line at the anterior end, as in 
Fig. 12, it is able to glide, but at a slower rate than 
normal. It performs the crawling movement in response 
to stimulation at the posterior end, and each half of the head 
performs feeling movements independently of the other half. 




Fig. 12.— Operation diagram. The heavy line indicates the cut 
made. For results see text. 

An individual slit up in the middle line from the posterior 
end, as in Fig. 13, glides at approximately the normal rate, 
provided the cut is not carried too far forward. If the cut 
extends into the head region the gliding becomes im- 
mediately slow^er. Such a specimen performs the crawling 



MOVEMENTS, KTO., OF FKESH-WATEK PLANAKIANS. 573 

nioveineut upon stimulation of the posterior end of either 
piece, but in a peculiar way, whicli will be described later. 

Putting all these results together, we see that there is a 
general tendency for animals on which operations have been 
performed to glide at a slower rate than normal. In some 
of the pieces this tendency is very slight, and frequently 
hardly noticeable. In others the movement is very much 
slower than normal. In all cases the periods of rest are 
longer during the time of regeneration than normally. Tiiis 
tendenc}^ for the animals to remain quiet during regenera- 
tion increases up to a certain point as regeneration proceeds. 
A piece of a planarian njay be quite active for three or four 
hours after the operation, while during the following three 
or four days it will scarcely move at all. After the regenera- 
tion is practically complete the worm will begin to move about 
again approximately as it normally does. During the re- 




JflG. 13. — Operation diagram. The heavy line indicates the cut made. 
For results see text. 

generating process the anterior pieces, bearing an uninjured 
head, are much more inclined to move about than are the 
posterior parts. These latter usually remain entirely quiet 
during regeneration. 

This behaviour of the posterior parts during regeneration 
appears to be distinctly purposive, and to belong to the class 
of phenomena called regulatory. The general tonus of these 
pieces is immediately lowered by the operation, and conse- 
quently they keep quiet. Yet at the same time the processes 
of morphallaxis and, in many cases, growth begin at once, 
and proceed very vigorously till the missing parts are 
restored. If we consider that the worm or part of a worm 
has at the beginning a certain sum-total of energy available 
for all activities, including movement, growth, morphallaxis, 
and all its other vital processes, then it would appear that 

VOL. 46, PART 4. NEW SKRIES. P P 



574 RAYMOND PEARL. 

the performance of any single set of activities in excess must 
cause a corresponding diminution in other activities. This 
is exactly what we find to be the case with the regenerating 
planarian. While the processes concerned in regeneration 
are at their masimiuu activity^ we get a decided reduction in 
the amount of movement. It would seem, then, that a large 
part of the energy which is ordinarily expended in movement 
is used after operatiou or injury in the processes of regene- 
ration. As the regeneration nears completion more and 
more energy is available for, and used in movement. This 
would seem to be a sort of "energy regulation." The 
behaviour is evidently further beueficial in the case of the 
posterior pieces, because their anterior ends are very insensi- 
tive as compared with the head of the normal animal, and 
if they moved about they would certainly be more apt 
in the long run to get into difficulties than if they remained 
quiet. 

It may be well in closing the section to point out the 
relation of the nervous system to the movements. Loeb 
(:00) has maintained that "if we divide a fresh-water 
planarian, for instance Planaria torva, transversely, the 
posterior half, that has no brain, crawls just as well as the 
oral half. Spontaneity in Planaria torva is, therefore, by 
no means a function of the brain." If by "crawl" in the 
first sentence we understand "glide" to be meant, the state- 
ment is not strictly accurate. The posterior pieces do not 
"move just as well as the oral" (anterior), but, as has 
already been brought out, more slowly. For a very short 
time after the operation the statement would in some cases 
be correct, but it certainly would not be twenty-four hours 
later, according to all the observations I have been able to 
make on the subject. As for the spontaneity of the move- 
ment, that also becomes very much lowered with the loss of 
the brain, as I have attempted to show above. The very 
much lessened activity of posterior pieces of planarians has 
been mentioned by Lillie (: 01, pp. 182, loo). 

From my own observations it seems clear that the principal 



MOVEMIiNTS, ETC., OF FliESH- WATI'JIi I'LANAIil ANS. 575 

function of the brain of PI an aria with reference to move- 
ments is to maintain the tonus of the ciliary system. That 
neither the crawling nor the gliding movements are specific 
functions of the central nervous system is evident, because 
both sorts of movement may take place after its removal. 
Yet all my observations tend to show that after injury to or 
loss of the brain the gliding movement becomes, almost 
immediately, markedly slower. This relation is especially 
well indicated by the experiments noted above on splitting 
the animal longitudinally from the anterior and the posterior 
ends. In the one case the gliding movement becomes at 
once distinctly slower, while in the other case there is only a 
slight difference in the rate, evidently conditioned by the 
fact that only comparatively few of the cilia can get a hold, 
so to speak, so that they can function. The force of the 
argument will be impressed if one glances at the relative si/e 
of the cuts in Figs. 12 and 13, and then remembers that the 
rate of gliding of the specimen figured in Fig. 13 is faster 
than that of the one in Fig. 12. With the co-ordination of 
movements, including the crawling, the ceutral nervous 
system has very little to do in the case of Planaria. With 
regard to the spontaneity of movement it is difficult to decide 
in how far the brain functions. It is certain that regenerat- 
ing antei'ior pieces show more spontaneous movement than 
do posterior pieces, yet the anterior pieces are behind the 
normal worm in this respect. The brain probably plays 
some part in the perforuiance of normal spontaneous move- 
ments, but, as has been pointed out, in these operation 
experiments the whole matter is very defiuitely related to 
the regenerative process, and loss of substance plays nearly, 
if not quite as great a part as loss of nervous system. 

Summarising, we may say that — 1. For the performance 
of the crawling or gliding movements the brain is not specifi- 
caliy necessary. These movements are normally co-ordinated 
in the absence of the brain. 

2. The maintenance of the tonus of the ciliary system 
(which produces the gliding movement) is a specific function 



576 EAYMOND PEARL. 

of tlie brain, and is, further, its most important function so 
far as movement is concerned. 

3. The brain plays a certain part in the production of 
spontaneous movements. 



F. Reactions to Stimuli. 
I. Eeactions to Mechanical Stimuli. 

Since the reactions which are given by Planaria to 
mechanical stimuli are in a sense the foundation on which 
the reactions to other stimuli are based, it may be well to 
consider them first. After thoroughly working out the 
reactions to mechanical stimuli we have a very definite clue 
to practically all the animal's behaviour. 

a. Methods. — For rough, general work with mechanical 
stimuli a needle or a sharp-pointed scalpel may be used as 
the stimulating agent. For the finer work in sharply local- 
ising the stimulus, I at first made use of pieces of glass 
tubing drawn out to capillary fineness. This method was 
not, however, satisfactory, as the glass was too stiff to admit 
of reaching all points of the body under some circumstances. 
Furthermore, this stiffness, together with the sharpness of 
the end, made it almost impossible to give the animal a 
moderately strong stimulus without wounding it. A far 
better plan was found to be to fasten with sealing-wax a 
moderately stiff piece of human hair to a piece of glass 
tubing, the latter to serve as a handle. With such an 
arrangement the stiffness of the stimulating point can be 
varied by varying the length of the hair. Danger of 
wounding the animal is avoided, yet repeated strong stimuli 
may be given, while, further, the flexibility of the hair makes 
it possible to stimulate the animal at any point and from any 
desired direction. 

An annoying difficulty in connection with this work was 
the clinging of the slimy secretion of the body to the point 
used for stimulating. Once coated with this slime the 



MOVEMENTS, KTO., OF FRESH-WATER FLANARTANS. 577 

sharpest point will slide off the body without givinc^ any 
effective stimulation. 

h. Description of Reactions. — The reactions can best 
be described by taking up in order the typical results" 
following stimulation of the different parts of the body. 

1. Stimulation of Head Region. — If a planariau 
gliding along on the bottom of a dish be touched with a 
needle on one side of the head, it Avill, under normal circum- 
stances, in the majority of cases, turn the head and anterior 
one fourth of the body away from the side stimulated, and 
continue gliding along in the new path determined by the 
turning of the anterior end. This 'burning away" reaction, 
or, as we may call it for economy of words, negative reaction, 
will always be given if the stimulus is made sufficiently 
strong. There is a certain intensity of stimulation below 
which the negative reaction may or may not be produced, 
depending on the physiological condition of the individual, 
but above which it always occurs. If, again, a normally 
gliding planarian be selected for stimulation, and this time 
the stimulating point (preferably something finer and more 
flexible than a needle) be touched very lightly to the 
edges of the sides of the head or the auricles, we get, 
provided the specimen is in the proper physiological con- 
dition, a very gi*aceful and striking reaction, quite different 
from that obtained in the former case. This time the flat- 
worm will stop for the briefest instant, turn the head and a little 
of the anterior end of the body towards the side stimulated, 
and at the same time raise the head from the bottom, until 
finally the tip of the head points exactly towards the point 
from which the stimulus came, and then glide forward in that 
direction. This "turning towards" or positive reaction is 
given only in response to very weak mechanical stimuli, and 
then only when other conditions are favourable. It is a very 
precise and characteristic reaction when it does appear. 

Having outlined the two main reactions following mechan- 
ical stimulation in the head region, we may proceed to con- 
sider each of them in more detail. 



578 



RAYMOND PEARL. 



a. Reactions to Strong Stimnli. — Tlie negative reac- 
tion is the characteristic reaction given to all strong stimuli, 
whether mechanical or of some other sort. It is, further, the 
same tjpe of reaction which most organisms with fairly well- 
differentiated reactions give in response to strong stimulation. 
It takes the animal away from what might be a dangerous 
object. 

In Planar i a the portion of the body which takes part 
in the turning away varies with the strength of the stimulus 
to a certain degree. Stimuli just strong enough to call forth 
the negative reaction will cause onl}^ the head to be turned 
away. The first turn away of the definite reaction never in- 




FiG, 14. —Diagram sliowing the form of the nen;ative reaction to mccliaii- 
ical st.iinuli. A shows the position just, bel'oie the slimuhis is 
applied, and P> the position after the reaction. 

eludes any of the body back of the pharynx, so far as I have 
observed, except in the case of very strong and repeated 
stimuli. In the typical and tn^st often observed form of the 
negative reaction the portion of the body which turns away 
is that anterior to a point about halfway between the level 
of the eyes and the point of origin of the pharynx. This is 
shown in Fig. 14. With stronger stimuli the poiut of turning 
is farther back on the body. 

The number of degrees through which the head is turned 
in the negative reaction depends on the intensity of the 



MOVEMENTS, ETC., OP FRESH-WATKR PLANARFANS. 579 

stimulus. Witli stimuli just effective iu calliug- forth the 
reaction the turn is only slight, and since it affects only the 
liead end the direction of movement of the whole animal ma}'^ 
be scarcely changed at all. The amount of turning of the 
anterior end is typically from 30° to 40°. 

There is in the negative reaction a pause at the instant of 
stimulation, pi-eceding the turning away. The first effect of 
the stimulus is to cause the animal to stop its relatively rapid 
movement. This pause may be so slight as to be almost 
imperceptible in the case of comparatively weak stimuli, or, 
on the other hand, may lengthen to a quite noticeable 
interval when the stimulus is very strong. It is a character- 
istic feature of both the positive and negative reactions of 
planarians, and is evidently duo merely to the fact that before 
a reaction (i. e. something involving a change of motion) the 
former movement must stop. 

The effect of localisation of mechanical stimuli in the head 
region may next be considered. As has already been men- 
tioned, stimulation of the sides of the head produces the 
positive or negative reaction according to the intensity of the 
stimulus. There are no special regions of specific sense- 
organs connected with either of these reactions. The nega- 
tive response is given after strong stimulation of any part of 
one side or the other of the head and, so far as it is possible 
to observe, just as decidedly after stimulation of one part as 
of another. It is of interest to know what happens after 
stimulation of the head iu the median line. It is very 
difficult to get a stimulus exactly in the median line, but one 
may come very near it by stimulating the dorsal surface of 
the liead in the region between the eyes. The reaction pro- 
duced is a longitudinal contraction of the anterior part of the 
body, drawing the head back away from the stimulus. The 
head is then turned to one side or the other as in the usual 
negntive reaction, and the animal starts ahead again in the 
new direction. The side towards which the turn is made 
after median stimulation is indeterminate — that is, there is no 
tendency to turn in more cases towards one side than towards 



580 EAYMOND PEARL. 

the ofclier, as has been found by Prandsen (: 01) to be the 
case with Li max. This is what would be expected in the 
case of the flat-worm, because it is a perfectly bilaterally sym- 
metrical organism. Probably what actually determines which 
way the organism shall turn after attempted median stimula- 
tion is the fact that the stimulus really acts a little to one 
side or the other, and the turning is really the negative 
reaction. 

By repeatedl}^ stimulating the anterior end of a worm with 
moderately strong mechanical stimuli its reactions may be 
modified. In the beginning of such a series of stimulation 
the worm turns away farther and farther from each suc- 
ceeding stimulus, at the same time remaining at the same 
place in the dish, i. e. not making any progressive move- 
ments. This process tends to make the animal describe a 
circle away from the stimulus, about its posterior end as a 
fixed point. It never completely describes a circle, how- 
ever, but after several stimuli have been given, to which it 
has responded progressively more vigorously, it finally 
jerks back with a strong longitudinal contraction, and turns 
the anterior end through a considerable arc, so that it points 
in an entirely different direction. This final strong reaction 
in the majority of cases turns the anterior end towards the 
side from which the stimulation is coming, or, in other words, 
in an exactly opposite direction to that of the previous 
reactions. This reaction appears as if, after the animal has 
tried in vain to get away from an uncomfortable stimulus by 
its ordinary reaction, it finally tries a wild jump in the oppo- 
site direction. This curious change in the i-eactions induced 
by a repetition of strong stimuli I have observed many times. 
It indicates the effect of the organism as a whole on its 
refiexes. As von Uexkiill (: 00, p. 73) has well brought 
out, we must consider that in the case of a higher organism, 
like a dog, the animal moves its legs, while with a lower 
organism whose activities are reflex — for example, the sea- 
urchin — it is really the " legs" (i. e. locomotor organs) which 
move the animal. In the flat-woi-m the movements of the 



MOVEMENTS, ETC., OP FRESH-WATER PLANA1?TANS. 581 

whole organism are determined by definite stereotyped 
reflexes, yet in such exceptional cases as the one just 
described the organism as a whole takes control, and does 
something quite different from what the normal reflex fitted 
to the case wonld accomplish. 

Very sti'ong mechanical stimulation of the anterior end, 
such as to wound the animal, causes a very much more 
vigorous reaction than the ordinary negative one, and of a 
slightly different form. The animal contracts strongly longi- 
tudinally, and, as a result of the heavier musculature on the 
ventral surface, curls the head in under the body. Then the 
anterior end is turned to one side through a larger angle 
than is usually the case, and the worm straightens out in this 
new direction. The point of importance to be noted in this 
reaction to maximal stimuli is the curling under of the head. 
The turn away from the side stimulated frequently is so great 
as to turn the animal squarely about, so that it heads in the 
dii'ection opposite to that before stimulation. Besides this 
effect of maximal stimuli just described, they may also pro- 
duce a change in the movement from gliding to crawling. 
The crawling does not usually follow stimulation of the 
head end of the body, but it is possible in some cases to pro- 
duce it by very strong stimulation here. I have also been 
able in a normal animal to induce crawling backward by 
very strong and continued stimulation of the anterior end of 
the body. This backward crawling, when it occurs, is of the 
same character as the same movement in a forward direction, 
except that all the factors are reversed. It has been 
described above (cf. p. 551). It is much more easily pro- 
duced after certain operative procedures, and in connection 
Avitli them further details regarding it will be brought out. 

The negative reaction, i. o. that to strong stimuli, is given 
more frequently than any other in the course of the activity 
of the individual, and apparently does not depend on the 
presence of an 3^ special ph^^siological condition. It is given 
in response to stimuli covering a wide range of intensit}^. 
The lower liminal value of the stimulus producing it (there 



582 RAYMOND PRARL. 

is appai'ently no upper limit) varies to some extent with the 
physiological condition of the individual. 'Julius in some 
specimens at certain times stimuli which would ordinarily 
produce a rather strong negative reaction will call forth 
nothing but the positive reaction. This condition is only a 
transitory one, and the reason for it seems to be a heightened 
tonic condition of the animal. Specimens exhibiting this 
relation to rather strong stimuli are always very active, and 
move about with great rapidity, frequently raising the 
anterior end of the body and waving it about through the 
water as they glide along. Persistent strong stimulation 
of the organism rapidly changes the general physiological 
condition. This is not more true of stimulation applied to 
the head region than of strong mechanical stimulation of any 
part of the body. The animal becomes '^ stirred up "gene- 
rally, moves about with increased rapidity, its sensitiveness 
to stimuli becomes diminished, and it will give only the 
negative response to stimulation of the anterior end. This 
change in the physiological condition of the animal as a 
result of continued stimulation of any sort, as in a series of 
experiments, is a matter of great practical importance in 
connection with reaction work. One may get totally differ- 
ent appearances from an individual Avhich has been " stirred 
up " from what are seen in the case of one which is in the 
normal condition. This is only one of a number of factors 
which must be taken into account in woi'k on the reactions 
and behaviour of an organism if one is to obtain trustworthy 
results. It is almost an absolute necessity that one should 
become familiar, or perhaps better intimate, with an organism, 
so that he knows it in something the same way that he 
knows a person, before he can hope to get at even an approxi- 
mation of the truth regarding its behaviour. 

j3. Reactions to Weak Stimuli. — The positive reac- 
tion is the characteristic reaction given to all weak stimuli. 
It is an orienting reaction in the sense that it brings the 
anterior end of the animal in a position such that it points 
approximately towards the source of the stimulus. On 



MOViaiENTS, ETC., OF FRESH-AVATRR PLANARFANS. 583 

account of it-^ fineness of adjustment witli refiM-ence to tlie 
strength of the stinuilus and tlie general pli^'siological 
condition of the animal, it is a response which might be 
very easily overlooked in a superficial examination of the 
behaviour. As the worm gives this positive reaction in 
response to a gentle stimulus, turning the head towards the 
source of stimulation, and at the same time raising it, it 
gives one the impression that it is seeking something, and 
such the behaviour would doubtless be called by some 
animal psychologists. This impression is enhanced by the 
fact that if the head does not come in contact with the 
stimulating object at the first reaction, the animal advances 
in the direction from which the stimulus came, with the 
anterior part of the body raised and waving from side to 
side in the water. 

As has been mentioned, the reaction is very delicately 
adjusted physiologically. In the majority of cases the 
animal must be in a comparatively quiet condition, — that is, 
not " stirred up ^' or excited, and gliding smoothly at the 
ordinary rate, in order that the reaction may appear at all. 
The stimulus must ordinarily be very weak, and given so as 
not to disturb the animal by abruptly changing the sur- 
rounding conditions. It is possible to produce the reaction 
by the use of a needle or scalpel point if sufficient care is 
taken, but better results are obtained by the use of a hair 
as the stimulating point. The point should be lightly touched 
to the edge or dorsal surface of the head, and then quickly 
drawn a short distance away. Even when all these precau- 
tions are taken one may fail to produce the characteristic 
response. I have frequently found that the same specimen 
which at one time would give the positive reaction in a very 
definite and characteristic way to every light touch on the 
head could not be made to show it a few hours later. This 
shows how closely it depends on general physiological 
conditions. On the other hand, specimens will frequently 
be found that for short periods of time (two or three hours) 
can hardly be induced to give any other response to median- 



584 RAYMOND PEARL. 

ical sfciraulation of tlie head. Stimuli strongs enong'h to be far 
above the usual upper limiual value for this reaction will call 
it forth. Such specimens show the reaction in a much more 
pronounced type than is usually the case. After a stimulus 
lias been given they will turn towards it, and if the source is 
not touched immediately they will remain in the same spot 
waving the head about the region from which the stimulus 
came, at the same time stretcliing the anterior end of the 
body far out in all directions, precisely as if in search of the 
stimulating body. Usually this hypei'sensitive condition 
passes off in a short time, and the animals behave again in a 
more normal fashion. It was thought that possibly this 
condition was due to hunger, but experiments^ devised to 
test this question indicated that this was not the case. 
We can only say that it is due to some intimate physio- 
logical condition, the exact nature of which we do not know. 
Another fact which may be mentioned in this connection is 
that sometimes a specimen in normal condition will give the 
positive reactioti in response to a certain strength of stimulus 
only a part of the time. Other trials I'esult in entire indiffer- 
ence on the part of the organism. Of course, it is not 
possible to give mechanical stimuli always of the same 
strength, yet with the closest possible approximation to this 
by an experienced operator, some of the trials will not affect 
the animal in any way except to cause a slight local con- 
traction at the point on the head stimulated. The worm 
glides along without any change in rate or direction. 
Altogether we must conchide that the reaction is one which 
is very closely dependent on the existence of certain definite 
internal conditions as well as the external ones. 

The typical course of the reaction is, as has been described, 
first a momentary pause, followed by a turning of the head 
towards the stimulus, accompanied by a raising of the ante- 
rior part of the body. From this typical form of the reaction 
there are many variations. The raising of the anterior end 
from the bottom just before and during the time it is being 
' See section on " Reactions to Fooii and Chemicals." 



MOVEMENTS, ET(J., OE FRESH- WATEli PLANAttlANS. 585 

turned towards the source of the stimulus may be entirely 
omitted. In this case the head is swept around towards the 
stimulus without being any further i-aised from the bottom 
than in the ordinary glide. The duration of the pause 
immediately following stimuhition is likewise subject to 
great variation. It may be so diminished as to be imper- 
ceptible, the worm sweeping the anterior end around through 
the water without any change in the rate of the glide. The 
amount of the turn varies with the point of application of the 
stimulus, the head being turned just far enough to point in 
the direction from which the stimulus comes. This orieuta- 
tion, if we may so call it, is generally quite exact. If the 
stimulus is near the middle line on the edge of the head the 
turn will be only through a few degrees, while if the auricles 
are touched it will amount to nearly 90°. This fact indicates 
the remarkably well-developed co-ordination of the reaction. 
There is a great deal of variation with regard to what takes 
place after the turn has been made, and the anterior end is 
directed towards the stimulus. If the stimulating point is 
removed immediately after stimulation, so that the animal 
does not touch it by means of the first reaction, a normal 
specimen will usually lower the head and continue gliding in 
the new direction. As has been mentioned, however, in 
some cases a specimen will continue " feeling" about in the 
locality for some time. If the stimulating point is held in 
about its original position after the stimulus luis been given, 
the first reaction will in most cases bring the head into 
contact with it. In this event the animal usually moves the 
tip of the head about over the hair (or other point) for a 
short time, and then drops back to the bottom and continues 
gliding. In other cases it will clasp the anterior end about 
the hair (as in the feeding reaction to be described later), 
and then in a moment start gliding up over it. When this 
happens the hair or needle may be moved about in the water 
or even lifted out of it, and the animal will not let go its 
hold and drop off. If the needle is held fjuiet, however, the 
animal will in a short time glide down off it and proceed on 



586 EAYMOND PEARL. 

its way along the bottom. This behaviour when the animal 
is able to reach the stimulating- object is evidently the action 
Avhicli most frequently occurs in natural environmental con- 
ditions. 

With reg'ard to the localisation of the stimulus producing" 
this reaction, I may say that I have been able to produce it 
by proper stimulation of any part of tlie edge or dorsal 
surface of the head region under favourable circumstances. 
It seems to be moi-e certainly produced — that is, in a larger 
number of cases — by stimulation of the auricles than of any 
other part of the head, and it may be that in this is to be 
found an indication of the chief function of these sense-organ 
bearing structures. At any rate, this is the only indication 
of a special function for them which I have been able to 
discover. The positive reaction given in response to light 
stimulation of the dorsal surface of the head is necessarily 
somewhat different from the typical reaction which has been 
described. In this case there can be no turning towards one 
side, because if this were done the liead would not be directed 
towards the source of the stimulus. Instead, what takes 
place is this : the head is sharply raised and twisted, so as 
to form a part of a spiral in the region posterior to the head. 
This brings the anterior end into a position pointing towards 
the source of the stimulus, and at the same time the ventral 
surface is brought around so as to be, in most instances, the 
tirst portion of the body to touch the stimulating point. 
This reaction, following stimulation of the dorsal surface of 
the head, is not an easy one to obtain. I have succeeded 
best in producing it in the case of individuals in the hyper- 
sensitive condition mentioned above. 

With regard to the strength of the stimulus necessary to 
call forth the positive reaction, only very relative statements 
may be made. Unfortunately Ave have no method of measur- 
ing the intensity of such weak mechanical stimuli as are 
used in work on lower organisms. Our only idea of the 
strength of the stimulus must come from the reaction of the 
organism itself. It must suffice to say, regarding the reaction 



MOVKMENTS, ETC., OF FRESH-WATER I'LANARIANS. 587 

uirIlt discussion, that iu au auinial in a condition of liypor- 
seusitivity I have been able to produce the reaction by the 
weakest stimuli whicli I was practically able to give. Under 
normal conditions of sensitivenes it takes a slightly stronger 
stimulus. No absolute value can be given for the upper 
limen of the reaction, beyond which it docs not appear, but 
gives place to the negative reaction. This value varies 
greatly with different individuals. The general statement 
may be made that the positive reaction is the characteristic 
response to stimuli of very low intensity, and its production 
is very closely dependent on the proper gradation in the 
intensity. This dependence is so close that it is possible to 
obtain a part of both the negative and positive responses 
combined in the same reaction by tlie use of a stimulus of 
the proper intensity. I have been able in a few very favour- 
able cases to produce by a single stimulus a pronounced 
raising of the head, such as is characteristic of the positive 
reaction, followed by a turning away from the source of the 
stimulus. Now the raising of the bead is no part of the typical 
negative reaction, and, furthermore, was done in the very 
characteristic way in which it occurs in the positive reaction. 
The stimulus whicli produced it was evidently about in- 
termediate in intensity between what, in the case of tlie par- 
ticular animal used, would have called forth either the positive 
or negative reaction, as the case might be. This experiment 
shows in a very striking and conclusive way that in both 
the positive and negative reactions we are dealing with a 
complex of reflexes, since here a part of one of the reactions 
is associated with a part of the other. This point will be 
alluded to again in another connection, and its significance 
more fully pointed out. 

The evident purposeful character of the positive reaction is 
plainly apparent. It is a reaction admirably suited, on the 
whole, to bring the organism into contact with beneficial 
things, such as food, etc. It seems to me that it nnist be by 
far the most important reaction of the animal in the struggle 
for existence. In the conditions under which planariaus 



588 RAYMOND PEARL. 

live a reaction which gets it foodj or helps to^ is of far 
greater importance for the survival of the individual than a 
reaction which takes it out of dauger; for, so far as observa- 
tion can show, the dangers it encounters are relatively few. 
It does not move over large areas of territory, and, so far 
as is known, it does not furnish a considerable part of the 
food supply of any other organism. Altogether its chief 
struggle for existence would seem to consist in obtaining 
subsistence for itself, and for this the positive reaction to 
mechanical stimuli would appear to be an important aid. 
As will be shown later, the food reaction proper consists 
largely of this same response. 

We may now pass to a consideration of the — 
2. Reactions to Stimuli applied to the Middle 
Region of the Body. — I use the term "middle region of 
the body " to distinguish that portion extending from the 
posterior border of the head to about the middle of the 
pharynx. The separation of the body behind the head into 
a " middle '^ and a " posterior " region is based entirely on 
physiological considerations, and is not defined morpho- 
logically. 

a. Reactions to Strong Stimuli. — Strong mechanical 
stimulation of the middle region of the body along the lateral 
edges causes, in the first instance, a local contraction of the 
body in the immediate region of the stimulus. This local 
contraction is well marked; much more distinct than that in 
the head region. If the stimulus is sufficiently strong, and 
especially if the stimulating point is applied to the edge 
from above rather than from the side, the previous gliding 
movement will be changed to crawling. This will continue 
for a brief interval, usually from two to four crawling con- 
tractions being given; then the animal will relapse again 
into the glide, provided the stimulus is not repeated. In 
the case of a strong stimulus applied to the side of the 
middle region of the body, especially if the stimulus is 
several times repeated, we get the negative reaction — a 
turning away from the side stimulated — just as in ihe similar 



MOVEIFENTS, ETC., OF FRESH-WATER FLANAIJIANS. 589 

case in tlie head region. The nearer to the anterior end of 
the middle region the stimulus is applied, the more easily 
will the negative reaction he produced, while back in the 
pharyngeal region it follows even strong stimulation in fewer 
cases. In all cases where this reaction is not produced the 
direction of movement is either unchanged by mechanical 
stimulation, or the anterior end may be brought around 
vei'y slightly towards the side stimulated as a mechanical 
i*esult of the local contraction on that side. Hy repeating 
the strong stimuli on one side of the middle region, summa- 
tion effects similar to those described above as taking place 
when the head is similarly treated are not produced. The 
animal crawls faster and faster away from the stimulus. Its 
direction of movement is changed, but usually not more than 
thirty to forty degrees. We see here evidence of precise 
response to localisation of stimulus. Stimulation of the head 
causes the animal to turn to one side, and, in case the 
stimulus is very strong, to contract longitudinally strongly 
before doing so. As we go back from tiie head in the 
middle region of the body, the tendency to crawl rapidly 
ahead away from the stimulus increases. At the same time 
the turning away from the stimulus becomes less and less 
marked the farther back it is applied. In no case do we get 
any strong retraction of the anterior end, which, in case of 
stimulation of the middle region of the bod}^, would tend to 
bring the animal, or at any rate a part of it, into further 
contact with the stimulus. 

Strong stimulation applied to the dorsal surface of the 
middle region of the body causes the animal to change from 
the glide to the crawl. This change of the form of motion 
may be regarded as a specific reaction in response to strong 
stimulation of the middle or posterior regions of the body. 
Stimulation of the dorsal surface of the middle region does 
not change the direction of the movement unless the stimulus 
is applied near the lateral margin, in which case it may 
cause the negative reaction, as mentioned above. 

/3. Reactions to Weak Stimuli. — Weak media nical 

VOL. 4G, PAUT 4. NEW SEUIES. Q Q 



590 RAYMOND PEARL. 

stimulation of tlie sides of the middle region of the body 
causes, in the first instance, a small local contraction at the 
place stimulated, without any effect on the general direction 
of movement of the whole organism. Under favourable 
circumstances, however, it is frequently possible to get a 
quite different result by the use of a weak stimulus on the 
lateral margins of this region. Specimens of Planaria may 
be induced to give the characteristic positive reaction 
described above by stimulation of a point as far back as the 
middle of the pharyngeal region. The stimulus inducing it 
is of the same intensity and character as that which will pro- 
duce the same result in the head region. 

Some experiments bearing on this point will be reported 
in full, on account of their important bearing on theoretical 
questions to be taken up later. These experiments were to 




Fig. 15. — Diagram to show the portion of tlie side of the body {a I) 
within wliich weak stimulation produces the positive reaction. 

test the effects of weak stimuli on the sides of the middle 
region of the body, especially with reference to the relation 
of the physiological condition of the individual to its 
reactions. The experiments were performed on large 
specimens of P. maculata, and the region of the body 
stimulated was that from a to h in Fig. 15. Most of the 
stimuli were applied in the region between the auricles and 
the origin of the pharynx. The stimuli were given by 
■moving the point of a fine scalpel along the bottom of the 
dish till it came into contact with the margin of the bod}^. 
In this way no general disturbance was produced. The 
attempt was made to make the stimuli of as nearly as 
possible the same intensity each time. The results were 
classed as positive, negative, or indifferent, according as the 



MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 591 

specimens gave the usual positive or negative reaction, or 
kept their course without change, showing only the local con- 
traction. The first series which I will report was on an 
individual which was in a condition of great excitation, 
moving about with more than normal rapidity, and generally 
'' stirred up." The results of twenty-three stimulations on 
this specimen were — 

Expt. I. Positive responses . , 1\ 

Tvr J.- p. Specimen in state of 

JNegative responses . 0,'- ^ 

Indifferent responses . 22J 

A similar experiment with another individual in an 
entirely normal unexcited state, gliding at a moderate rate, 
gave the following results : 

Expt. II. Positive responses . 20^ 

AT i- o Specimen in normal 

JNegative responses . 2 V ^ 

Indifferent responses . ISJ 

The striking preponderance of the positive reaction in the 

case o£ the unexcited individual is notable. The same 

individual used in Experiment II was now " stirred up" by 

poking it violently about the dish with a needle for about 

five minutes. It was then allowed to settle into a glide 

which was at a more rapid rate than normal, and another 

series of stimulations was made, with the following results : 

Expt. II r. Positive responses • ^1 

XT i- o Specimen in condi- 

Negative responses . 3 ^ ' . 

T :i-cc i. o tion of excitation. 

Indinerent responses . 8; 

Here, again, the indifferent responses are in excess, and 
there are practically no positive reactions. The specimen 
was again " stirred up " in the same way as before, and 
another series taken. 



Expt. IV. Indifferent responses (trials 
1 to 11 inclusive) 
Positive responses (trials 12 

and 13) 
Negative responses (0) 



Specimen in state of 
excitation. 



592 RAYMOND PEARL. 

The specimen was ag-nin stirred np in the same way and 
another series taken, with the following resnlts : 



Specimen in state of 
excitation. 



Expt. Y. Indifferent responses (trials 

1 to 9 inclusiv^e) 
Positive responses (trials 10 

and 11) 
Negative responses (0) 

The positive responses in all these experiments were very 
definite and characteristic. I have obtained the same results 
in many other series of experiments, which need not be 
recorded in detail. The experiments show very clearly that 
in order for the animal to give positive responses to weak 
stinn;li it is necessary that it be in an unexcited condition. 
These resnlts have also an important bearing on the question 
of the mechanism of the positive response, in that they show 
conclusively that the reaction does not depend on the stimu- 
lation of special sense organs located in the head regions 
alone. 

Weak mechanical stimulation of the dorsal snrface in the 
middle region of the body is usually without any effect other 
than the causing of a slight local contraction at the point 
stimulated. If any specific effect on the whole animal is 
produced, it is merely a change from the gliding- to the 
crawling movement^ such as results from strong stimulation 
in the same region. 

3. Reactions to Stimuli applied to the Posterior 
Region of the Body. — By '^posterior region of the body '^ 
I mean that part of the body from the pharyngeal region to 
the posterior end. This region is not sharply marked off 
physiologically from the middle region, and it is impossible 
to say in any given individual at just what level the demar- 
cation will be found. The physiological distinction between 
the two regions is founded on the fact that it is possible by 
unilateral stimulation of the middle region of the body to 
produce a change in the direction of the movement of 
the animal as a whole, while in case of the posterior region. 



MOVEMENTS, ETC., OE FUESH-WATEK I'LANAKIANS. 593 

as will be showily this canuot be done. On tin's uccounfc it 
will not bo necessary in the description of the reactions to 
sliarply distinguish between the effects of stimulation of the 
margins and of the dorsal surface^ as has been done in the 
previous cases. 

Strong mechanical stimulation of the posterior region of 
the flat-worm produces as a specific reaction an immediate 
change from the gliding to the crawling movement. The 
direction of the crawling is the same as that of the gliding; 
that is to say, the worm keeps on in a straight lino, taking 
itself directly and in the quickest possible way away from 
the stimulus. The duration of the crawling movement 
following stimulation of the posterior region varies with the 
relative intensity of the stimulus and the physiological con- 
dition of the specimen. The most usual number of the 
strong, crawling contraction waves following strong stimula- 
tion is three or four. We may get a smaller number than 
this, and very frequently do, but iu the species studied I 
have very rarely seen more than four of the general con- 
tractions following a single stimulus. This is evidently all 
that would be necessary under normal circumstances, since 
four of these strong contractions will carry the animal a 
considerable distance ahead, and probably out of reach of 
the stimulating agent. The weaker the stimulus is, the fewer 
are the contractions and the shorter the distance crawled. 
In some individuals it is at times almost impossible to induce 
the crawling movement except by repeated stimulation. 
Such specimens will merely draw up the posterior end in a 
single crawling contraction after stimulation, and then im- 
mediately relapse into the glide. If a strong stimulus is 
repeatedly given at the posterior end the crawling is con- 
tinued, becoming more and more rapid. This is the only 
effect of continued stimulation in this region, there being no 
summation effect corresponding to that produced by stimu- 
lating the anterior eud. No different effect is produced by 
stimulating the margins of the posterior region of the body 
from what takes place when the point stimulated lies near 



594 RAYMOND PEAUL. 

the middle line. There is no turning towards or away of 
any part of the body. The lack of any special effect of 
unilateral stimulation is not surprising, for the reason that 
rapid movement in a forward direction will get the animal 
away from harmful stimuli affecting this region, in the long 
run, more quickly than any other. Further, there would be 
no advantage in the production of a positive reaction by 
stimuli at the posterior end. If we think of these reactions 
as having been developed by natural selection there would 
be no possibility of such a reaction having arisen, for the 
reason that practically any favourable stimulus would be 
encountered by the anterior end before it possibly could be 
by the posterior. Very weak mechanical stimulation of the 
posterior end of the body causes only a local contraction at 
the point stimulated. 

4, Reactions to Stimulation of the Ventral Sur- 
face. — In the descriptions of the reactions to mechanical 
stimuli up to this point we have been considering stimuli 
applied to the dorsal surface and to the margins of the body. 
It may be well to describe briefly what the reactions in 
response to localised stimulation of the ventral surface are. 
This matter can best be tested when the animal is moving on 
the under side of the surface film, with its ventral side 
uppermost. It might be supposed before the trial was made 
that this habit of the animal would afford ideal conditions for 
testing its reactions to ventral stimulation, but, as a matter 
of fact, the conditions are anything but ideal. The flexibility 
and elasticity of the surface film makes it almost impossible 
to touch it with a stimulating point anywhere within a radius 
of a centimetre about a planarian without causing the animal 
to be jerked bodily to one side or the other, quite sharply 
and for some little distance. This is, of course, a mere 
mechanical effect, which takes place with lifeless bodies also. 
Furthermore, as has been mentioned in an earlier section, it 
appears to be very difficult for planarians to quickly change 
the direction of their movement when on the surface film (as 
is necessary in reacting to stimuli). On account of these 



MOVEMENTS, ETC., OF FRESH-WATER PLANARTANS. 595 

conditions it is very difficult to got any certain and trust- 
worthy results from the stimulation of the ventral surface. 
My results have been as follows: — strong stimulation of the 
anterior end on one side of the middle line causes the 
negative reaction just as when the stimulus is applied at a 
corresponding point on the dorsal surface. For mechanical 
reasons the response is not as extensive as when the animal 
is on a solid, but there seems no doubt of its character. The 
positive reaction to weak stimuli I have not been able to 
produce in any certainly recognisable form in response to 
stimulation of the ventral surface, but I think this negative 
result is due probably to the external conditions, and not to a 
real failure of the organism to react. Strong stimulation of 
the posterior end of the body causes the gliding to change 
to the crawling just as under other conditions. Very 
strong mechanical stimulation of the ventral surface of the 
body causes the animal to let go its hold and pass down 
to the bottom. 

5. Reactions of Kestin g S pecimens to Median ical 
Stimuli. — A resting specimen gives no response whatever 
to weak stimuli which arc still strong enough to produce a 
definite reaction when the worm is in the active condition. 
The stimulus is simply below the threshold of the resting 
animal's sensitiveness. To stronger stimuli the reactions 
correspond in form with those given by the active animal, 
but are less pronounced. For example, rather strong stimu- 
lation at the anterior end induces a weak negative reaction ; 
similar stimulation of the posterior end sets the animal off 
into the crawling motion. Strong stimulation of any part of 
the body besides producing the characteristic reaction for 
that region (that is the negative reaction) will also in most 
cases start the animal into movement. This will always be 
the case if the stimulus is of sufficient strength, or is several 
times repeated. As would be expected from the low sensi- 
tiveness of the resting flat-worm, it is impossible to call 
forth from it any positive reaction. 

G. Reactions to Stimuli given by Operative Pro- 



596 RAYMOND PEARL. 

cedure. — Evidently wlieu a plauarian is cut tlie cutting 
induces a strong stimulation, wliicli is of the same kind as 
that induced by ordinary mechanical stimuli, only much 
more intense. The immediate effects of operations may then 
be taken up in this section. 

If we take first the typical case given by cutting the 
animal transversely in two in the region between the pos- 
terior border of the head and the origin of the pharynx, and 
make the cut by a single stroke of a sharp scalpel, we find 
that the effect on the anterior piece is precisely the same as 
that of an ordinary strong mechanical stimulation of the 
same place. That is, this piece merely changes from the 
gliding to the crawling movement, and after giving three or 
four crawling coiitractions settles down again into the 
glide. This is the same result essentially as that obtained 
by Norman (: 00) and earlier by Loeb ('94 and : 00). In the 
behaviour of the posterior piece in this experiment under 
discussion there is a great deal of variation. In about 70 
per cent, of all cases in which I have observed the results of 
such an operation, the posterior piece crawled backwards 
as a result of the cut. In the remainder of the cases the 
piece either stayed in the same place and contracted 
violently, or else glided ahead. The amount of the back- 
ward crawling when this occurs varies greatly, from a 
short distance involving only one longitudinal crawling con- 
traction to several times the length of the worm, the move- 
ment lasting in this latter case for over a minute. In order 
that this backward crawling may appear in a well-marked 
and distinct form it is necessary that the posterior piece be 
above a certain size. Very small posterior pieces after 
operation usually remain quiet. 

A cut so made as to split the anterior end of the body in 
the middle line in most cases causes the worm to crawl back- 
wards just as does a transverse cut. In some cases this, as 
well as other operations, merely causes the animal to contract 
violently and squirm about at the same place. Splitting the 
posterior end of the body in the middle line causes the parts 



MOVEMENTS, ETC., OF FRESH-WATKll PLANAUIANS. 597 

on either side of tlio cut to give violent lougitudiual con- 
tractions, while the worm as a whole starts crawling ahead ; 
that is, it changes from the gliding to the crawling 
movement. 

Oblique cuts produce essentially the same effects as would 
trausvei'se cuts in the same part of the body, i. e. forward 
crawling of the anterior piece, and usually backward crawling 
of the posterior piece. This is true unless the cuts are very 
oblique, so as to form very acute angles with the sagittal 
plane of the body. In such cases the effects produced more 
nearly resemble those obtained in complete longitudinal 
splitting of the body. If the body is split completely into 
two parts longitudinally, there is usually very little pro- 
gressive movement of either piece afterwards. The pieces 
contract strongly on the cut sides very soon after the opera- 
tion is performed, so that they take on the form of a bow, 
which in many instances becomes a nearly complete circle. 
This being the case, any progressive movement, either by 
gliding or crawling, is nearly or quite impossible. Cuts 
involving only a small portion of one side of the body 
produce, if in the anterior region, the characteristic negative 
reaction given to other strong mechanical stimuli, while if in 
the posterior region they cause the crawling ahead. 

Cuts made on the resting animal produce essentially the 
same effects as on the gliding specimen. Unilateral cuts 
have the same effect in producing the negative reaction. 

7. The Effect of Mechanical Hindrance to Move- 
ment. — A series of experiments was performed on Dendro- 
coclum, sp., with reference to the behaviour of the animal 
when progressive movement was made impossible, and yet 
the animal was stimulated strongly at the same time. These 
conditions can be realised by thrusting a needle through the 
centre of the body from above, and then holding it fixed in 
position. The results of this procedure varied somewhat, 
according to the portion of the body through which the 
needle was thrust. In case the hindrance is in the posterior 
region of the body, e. g. at a point just behind the posterior 



598 EAYMOND PEARL. 

end of the pharynx^ the effect immediately following the 
thnistiiig iu of the needle is a strong longitudinal contraction 
of the whole body. After this first strong contraction the 
animal remains perfectly quiet in the contracted form for a 
varying length of time (in some cases as long as five minutes, 
but usually less). After this period of quiet a series of 
rhythmical waves of contraction pass longitudinally over the 
still contracted body. The purpose of these waves is 
evidently to loosen the restraining object by making the 
hole in the body through which it passes lai'ger. This is the 
same behaviour that I have observed in the deposition of the 
large egg. This process of rhythmical longitudinal contrac- 
tion is continued for a time; then the animal stretches to its 
extreme lengthy attaches the anterior end to the substrate, 
and attempts to crawl away. The movement of the anterior 
end is precisely the same as in crawling. The animal turns 
and twists and struggles violently in this attempt to crawl 
away, and the cilia beat strongly. If the needle occupies a 
position near the edge of the body this first struggle will 
usually be sufficient to tear the body loose from the needle, so 
that the animal may then move ahead freely. Such specimens 
will, of course, have a large jagged wound in one side of the 
body, which, however, closes in and heals in a short time. In 
case the first struggle of the extended animal to craAvl ahead 
is not effective, that is if the needle is too far in towards the 
centre of the body to make the tearing out possible, the 
animal, after continuing the struggle for a time, contracts 
strongly longitudinally and goes through the whole series of 
stages of quiet, rhythmical, longitudinal contraction and 
attempted crawling again. The only difference between the 
first and succeeding series of trials is that the stages in which 
the animal is strongly contracted longitudinally tend to 
become shorter with each repetition. 

In case the needle is thrust through the body in front of 
the pharynx, the strong longitudinal contraction appears as 
before, and is followed after some time by an extension of the 
part in front of the needle, while the rest of the body re- 



MOVEMENTS, ETC., OP FliESH-WATEU PLANARIANS. 599 

muins quiet iiiid coutracted. This short anterior region, 
including- hardly more ' than the head, goes through the 
crawling movements, but on account of its small size is very 
ineffective so far as pulling tiie body away from the needle 
is concerned. In my experiments I have never seen any 
worm succeed in getting free from a needle put through the 
body in this position. 

This general behaviour of the animal in response to restraint 
of movement is very interesting, especially in the cases where 
the restraint is at the posterior end, as showing the relation 
between the behaviour and the capability of regenerating. 
The organism tears itself loose from a restraining body with 
entire nonchalance, as it were, and its confidence is well 
founded because no permanent harm comes from the action. 
The lost and wounded parts are regenerated and healed in a 
short time. The behaviour takes advantage of the ability to 
regenerate. Whether the form of behaviour (pulling away 
from restraining objects) or the power of regeneration and 
reparation appear in the organism first we cauuot say, for 
either might very well follow, in a more or less remote causal 
connection, the other. What we do know is that at present 
there is a very nice condition of mutual adaptation between 
the two things. 

The effect of the hindrance of a rather light weight at the 
posterior end of a worm is to induce the crawling movement. 
This can be seen in case the animal is feeding on a small 
piece of food material, and, as frequently happens, starts into 
movement before the pharynx is withdrawn. The piece of 
food attached to the end of the pharynx is dragged along 
behind, and the movement is the crawling. Frequently, also, 
in feeding experiments pieces of food will get stuck to the 
posterior end of the worm by means of the mucous secretion 
of t])e body, and these have the same effect in inducing the 
crawliug movement. 

Having now obtained a descriptive basis we may pass to a 
discussion of some general features of these reactions. We 
may first take up — 



600 RAYMOND PEARL. 

c. The General Features of the Reactions to 
Mechanical Stimuli. — From the above description it 
appears that the nature of the reactions to mechanical stimuli 
depends upon several factors. These are — 

1. The intensity of the stimulus. 

2. The localisation of the stimulus. 

3. The physiological condition of the organism. 

The reactions given may be of several different kinds, de- 
pending on the factors mentioned above. These are chiefly 
as follows : 

1. The resting individual may begin locomotion. 

2. The gliding movement may be changed to the crawling 
movement. 

3. The forward movement may be transformed to move- 
ment backward. 

4. The animal may turn away from the source of the 
stimulus (the "negative " reaction). 

5. The animal may turn towards the source of the 
stimulus (the "positive " reaction). 

It is evident that the reactions last named — the negative 
and positive reactions — are the most important and most 
interesting from the theoretical standpoint. It is of the 
greatest interest to note that these two qualitatively opposite 
reactions are induced merely by differing intensities of 
stiumli, the stimuli being otherwise identical throughout. 

It is to be noted further that the positive and negative 
reactions have the characteristics of purely reflex acts. Each 
reaction has a perfectly definite and characteristic form. 
While, in some cases, which of the two reactions will be 
given in response to a particular stimulus depends on the 
physiological condition ot" the organism, yet it is practically 
always either one or the other of the typical reactions. Only 
very rarely do we get any deviation from the type forms, 
and in such cases the reaction is evidently a combination of 
easily recognisable components of the two typical complexes 
of reflexes. 

These two reactions are evidently not single simple 



MOVEMENTS, ETC., OF FRESH-WATKR PLANARTANS. 601 

reflexes, but arc coiii])lexes of several simple refJex acts. It 
may be well to present in tabular form the different com- 
ponents in each of these reactions, indicating- by the position 
in the table the relations of the parts. 

Component Phases of the Reactions to Mechanical 
Stimuli, with special reference to the Head 
E. e g' i o n . 

rosiTivi';. Nkoativk. 

A. Moiueutary stopjjiiii,' of pre- A. Same as in i)OMliv('. 

vious movement. Referred 
to as " pause " or " lusita- 
tiou " ill description. 

B. Longiludinal ext.ensioii of the B. Longitudinal contraction of ante- 

anterior end to greater or rior end of greater or less 

less extent. Amount de- intensity. Tends to make A 

pends on previous extension. appear more pronounced ar.d 

Usually distinctly noticeable. longer in duration. 

C. Turning towards one side, viz. C. Turning towards one side, viz. 
that stimulated. Tliis side that not stimulated. Defined 
is defined by the [josilion of as in positive. No sharp 
the source of the slimu- " orientation." 
lus, not structurally. Sharp 
"orientation." 

C. Raising of anterior end. This 
takes place at the same time 
as C. 

D. Movement towards stimulus. 1). Movement away from stimulus. 

Direction determined by Direction determined as in 

position taken l)y anterior positive, 

end at termination of C. 

Time relations arc indicated by vertical position in the table. Components 
occurring at the same time are included in braces. 

Each of the components before D may be considered as a 
sinofle reflex, and thus there are in one case four and in the 
other case three simple reflexes which go to make up the 
whole reaction. That these reactions are composites of the 
distinct parts is evidenced, lirst, by direct observation of the 
reactions themselves ; and second, by the fact that it is 



602 RAYMOND PEARL. 

possible by varying tlie strength of tlie stimulus to produce 
only certain parts of the whole reaction without the 
remainder, and, furthermore, that a part of one reaction may 
in rare instance be combined with a part of the other 
(v. sup., p. 587). 

d. Mechanism of the Reactions. — A question which 
is of the greatest iniportauce in all work on the reactions of 
organisms is, what is the mechanism of the reaction ? In 
the case of the flat-worm this becomes, what is the neuro- 
muscular mechanism of the reactions ? Very little direct 
evidence bearing on this question can be obtained from the 
reactions themselves. Taking the positive and negative 
reactions as they occur, there are several different sets of 
muscles and of nerve connections by means of which they 
might conceivably be brought about. The best evidence on 
the question is the indirect evidence from operation experi- 
ments, in which parts of the mechanism are injured or 
removed. 

1. Relation of the Brain to the Reactions. — The 
first specific problem which may be taken up may be stated 
thus : is the brain necessary for the performance of the 
normal reactions to mechanical stimuli ? Or, in other words, 
will a plauarian from which the brain has been removed 
react normally to stimuli ? This question can be answered 
from the study of specimens which have been cut in two 
transversely, and consequently we may proceed at once to a 
description of the reactions of the pieces resulting from such 
an operation. A typical specimen is cut in two transversely 
at the level of a point about halfway between the head and 
the origin of the pharynx, as shown in Fig. IG. As has been 
mentioned above, the cut itself acts as a strong mechanical 
stimulus, and the immediate effect of the operation is to set 
both pieces crawling, the anterior one ahead and the 
posterior one usually backward. 

If now the pieces are allowed some hours to recover from 
the immediate effect of the operation, and then stimulation is 
tried, the following results are obtained : — With the anterior 



MOVEMENTS, ETC., OF FREST[-WATER PLANARIAXS. 603 

piece A, containing tlie bi'ain, the results are entirely similar 
to those obtained in case of tlie normal animal. Stron<if 
unilateral stimulation of the head causes the negative 
I'eaction, weak stimulation of the same sort the positive 
reaction. Stimulation at the posterior end causes the crawl- 
ing movement to appear, and altogether the appearances are 
essentially the same as in the normal complete specimen. 

The posterior piece B (lacking the brain) behaves in a 
somewhat different manner. If the anterior end of this piece 
is given a stimulus of moderate intensity anywhere on the 
cut surface the piece will usually start crawling straight 
backwards. This is almost always true for a short time 
after the operation, and is especially well shown in such 
specimens as started crawling backwards as a result of the 
cut. AVhen from twenty-four to forty-eight hours have 
elapsed after the operation this tendency of posterior pieces 





Fig, 1G.— Operation diagram. Heavy line indicates cut. 

to crawl backward on stimulation of the anterior end begins 
to grow less marked, and, as regeneration proceeds, finally 
disappears. In many such posterior pieces I have been able 
to produce this backward crawling in a very pronounced 
form, and of comparatively long duration (three or four 
minutes at a time). The chai*acter of the movement has 
been described above. If the stimulus is applied to one side 
or the other of the anterior end of such a posterior piece, 
instead of squarely against the cut surface, a well-marked 
negative reaction is produced ; that is, the anterior end 
turns away from the stimulus just as a whole animal would. 
The reaction is very definite, and of precisely the same 
character as the normal negative reaction. The only 
difference to be observed is that in proportion to the strength 
of the stimulus the reaction is not so pronounced as in the 



604 RAYMOND PEARL. 

uormal animal^ this being due to the generally lowered tonus 
in such a piece. I have not been able to obtain any positive 
reaction (i. e. turning towards the stimulus) in sucli a 
posterior piece after operation. Stimuli which are at all 
effective produce the negative response. This experiment 
has been tried many times, but always with the same result ; 
the positive reaction never appears. If the posterior end of 
such a posterior cut piece is stimulated the crawling move- 
ment is produced just as in case of the normal complete 
animal. As has been noted in connection with the move- 
ment, there is a general reduction of tonus in the posterior 
pieces resulting from transverse cuts. This low tonus in- 
volves not only the motor functions, resulting in slower 
movement, but also to a less extent the sensory functions. 
Such a piece is somewhat less sensitive to mechanical stimuli 
than normally. The cut surface is more sensitive to 
mechanical stimuli than any other part. 

Now it will be seen from the above description of the 
reactions of a piece from which the brain has been removed, 
that the most striking difference in the behaviour of such a 
piece from that of a normal animal is to be found in the 
absence of the positive reaction. 

There are three conceivable possibilities as to the cause of 
the absence of the positive reaction in pieces from which the 
head has been removed. First, the positive reaction might 
be due to the stimulation of certain sense organs which are 
removed by the operation. But this is decisively negatived 
by the fact that in an entire worm stimulation of points 
posterior to the level of the cut removing the anterior end 
will cause the positive reaction. 

Second, it might be conceived that the reaction is brought 
about by a special localised muscular mechanism, which is 
removed or destroyed by the cut. But there is no evidence 
of the existence of such a mechanism ; and furthei-, it will be 
shown later that the ordinary musculature of the body, which 
is of course uninjured in the posterior part, is sufficient to 
brino- about the reaction. 



JJOVEMUNTS, ETC., OF l'liE8il-\VATEK PLANAUl ANS. 605 

Finally, the positive veuctiou might in sonic way be a 
specific function of the brain, which is removed by the 
operation. As the evidence seems to be decisive against the 
first two possibilities this seems probably true. Is this 
because the brain contains a special " centre " whose function 
it is to produce the reaction ? 

There is no reason to think of the reaction as a function of 
the brain in the sense that that organ forms a centre which 
originates the impulses Avliich cause the reaction. On the 
contrary, it seems much more probable that the loss of the 
brain causes the loss of reaction for the following reason. 
It has been shown that removal of the brain causes a general 
lowering of the tonus of the organism, and further that the 
appearance of the reaction in a normal animal is closely 
dependent on the tonic condition of the organism. Probably, 
then, the chief reason for the non-appearance of the positive 
reaction in posterior pieces is that in these the conditions of 
general tonus are so changed by the loss of the brain that the 
reaction is no longer possible. Expressing it in another way, 
the animal is too sluggish to give the positive response. 
This being the case, it would be expected that it might be 
possible to induce the positive reaction in a decapitated 
specimen provided the tonus were raised in some way. • As a 
matter of fact, as will be shown later, positive reactions to 
certain chemical stimuli have been observed in a few cases 
(cf. p. 649). In its form and mechanism the positive reaction 
is not directly dependent upon the brain. 

Summing up the evidence on the relation of the brain to 
the reactions of the flat-worm, it may be said that all the 
reactions to mechanical stimuli shown by the normal animal, 
with the single exception of the positive reaction, are given 
by specimens from which the brain has been removed. 
The relation of the brain to the positive reaction is, in large 
part, so far as evidence can be obtained, an indirect one, viz. 
it is necessary for the maintenance of the proper tonic 
conditions of the organism. Thus far there is no evidence of 
any special " centre " functions of the brain, similar to those 

VOL. 46, PART 4. — NEW SERIES. R K 



606 RAYMOND PEARL. 

supposed to exist in the cortical centres, for example, of a 
mammal. 

2. The Neuro-muscular Mecliauism. — In the negative 
reaction to mechanical stimuli the anterior end of the body 
is turned sharply away from the source of the stimulation, 
while in the positive reaction it is equally sharply turned 
towards the source. These relations immediately suggest the 
following questions : — Is the negative reaction the result of a 
crossed impulse, which, originating at the point stimulated, 
crosses over to the other side of the body and causes the 
contraction of the longitudinal muscles on that side, thus 
producing the turning away from the stimulus ? What is the 
course of the nerve impulse which produces the positive 
reaction ? What sets of muscles are concerned in the pro- 
duction of each reaction ? 

The discussion of the negative reaction may be taken up 
first. If the nervous impulse pi'oduciug this reaction 
crosses the body to produce a contraction on the side 
opposite from the stimulus, the experiment cited in the 
section above shows that this crossing cannot occur entirely 
in the brain^ but must also occur in some part of the body 
posterior to the brain ; or at any rate, be capable of so doing 
in a quite normal fashion immediately after removal of the 
brain. In this experiment where the body has been cut 
in two behind the brain, the posterior piece performs the 
negative reaction in a quite normal way immediately after 
the operation. This experiment may be carried farther, 
and the animal cut in two transversely in places nearer 
and nearer to the posterior end of the body. In all of these 
cases, until the piece becomes too small to show definite 
movements of any sort, the negative reaction may be obtained 
by strong unilateral stimulation. This shows conclusively, 
then, that if the negative reaction is to be considered a 
crossed reflex, there must be all along the body a series of 
cross-commissures which are at all times ready to bring about 
in co-ordinated perfection a result with which they have never 
])reviously had anything to do. This conclusion seems in- 



MOVEMENTS, ETC., OF FUESH-WATER TEANAUIANS. 607 

ovitablo because, as has been shown above, uiiihitei'al 
sfciniuhitioii of the posterior region of the body in a normal 
individual does not cause the negative reaction, but instead 
merely causes the animal to move ahead, faster by crawling. 
If these paths for the crossing of impulses which are so 
immediately effective after the operation are present in the 
uninjured specimen, one would expect the reaction to be of 
quite a different character from what actually occurs. A 
stimulus applied near the posterior end would naturally cross 
over at once and produce a bending on the opposite side at 
the same level. Or the stimulus might diffuse, so that the 
entire op])osite side Avould bo affected and the worm would 
become uniformly curved on that side. But as a matter of 
fact we find that the turning affects only the anterior portion 
of the body. If it is urged that after operation the crossing 
of impulses takes place through the general protoplasm the 
difficulties encountered are no less, for it must be shown how 
passage of an impulse through the protoplasm to cause a 
perfectly well co-ordinated reaction can appear so quickly and 
produce such perfect results at once. If tested immediately 
after the operation, before the general lowering of tonus is 
felt, the reaction time for the negative response of a posterior 
piece of the body will not differ appreciably from that of a 
normal worm. Now, according to the views of the advocates 
of the theory that after operations involving loss of nervous 
tissue, impulses may be conducted through the general 
protoplasm, it is held that such conduction is always at first 
appreciably slower than in nervous tissue. It would also 
seem on purely a priori grounds that this must be true. 

Thus it is seen that there are serious objections to the view 
that the negative reaction is the result of a contraction on the 
side of the body opposite to that stimulated — that is, that it is 
a crossed reflex. The question now arises, if the reaction is 
not produced in this way, in what other way can it be 
produced ? Evidently it is quite possible that the anterior 
part of the body can be turned away from the stimulus by a 
lengthening of the side stimulated, quite as well as by a 



608 



RAYMOND PEAWL 



sliorteuing or contraction of the opposite side. We may now 
consider the evidence as to whether or not the turning away 
is actually due to a lengthening of the side stimulated. 

Very little evidence can be obtained regarding this from 
observation of the normal moving animal, because the 
general appearance in the turning would be the same 
whether it were due to a shortening of one side or a 
lengthening of the other. The results from certain sorts of 
operation, however, give definite evidence on the question. 

A specimen split longitudinally in the posterior end, as 
shown in Fig. 17, a, and the cut was extended forward to the 
posterior border of the head region. Several days were 






Fig. 17. — «. Operation diagram, h. Showing side A supported on 
B. For further explanation see text. (Tiie pliarynx is omitted 
for the salve of clearness.) 

allowed for recovery from the shock of the operation, care 
being taken to prevent the two parts from growing together 
again. By this time the cut edges had healed well, and the 
specimen was in good condition for experimentation. The 
results of mechanical stimulation were as follows: strong 
stimulation of the head or anterior part of the body on either 
side caused the negative reaction; the anterior end turned 
away from the stimulus. But it was possible to tell in this 
case which of the two pieces or halves of the body were 
effective in producing the turning. It could be seen clearly 
that the half stimulated, immediately on stimulation, flattened 



MOVEMENTS, ETC., OF FRESH-WATER PLANARTANS. GOO 

out slig-litly ventrally, thus bringing the ventral cilia in close 
contact with the bottom, as is necessary for their effective 
working. At the same time it lengthened along its outer 
side, thus forcing the anterior end around towards the side 
opposite from the stimulus. That the '' side opposite " had 
nothing to do with the turning could be observed in many 
cases directly, for this side (B) would remain in an almost 
entirely relaxed condition after the stimulus was given, and 
not get any effective hold on the bottom so that it could 
affect the movement. It was further possible by a little 
manipulation to get the piece B laid over on A so as to be 
practically entirely supported by it, as shown in Fig. 17, h. If 
with such conditions the worm was stimulated rather strongly 
on the A side of the head, it gave a strong negative reaction, 
the poiiit about which the turn was made being as far back 
as X. Evidently with part B up on the dorsal surface of A, 
and consequently having no hold on the bottom, it could 
have no effect in the reaction. The reaction must have been 
due to the side A alone. The same thing could be shown by 
very gently lifting on a needle the side B so that it svas not 
in contact with the bottom, and then stimulating A, when 
again the negative reaction occurred. This experiment I 
have repeated with variations many times, but always with 
the same result, showing that the side stimulated is the 
effective one in producing the turning. 

It may be mentioned here that the effect of strongly 
stimulating the posterior end of either of the two pieces of a 
specimen slit in this way was to cause a local contraction of 
the piece stimulated, and a crawling movement of the short 
portion of the body in front of the slit. This crawling was 
not very effective, since so small a portion took part in it, 
but it is of interest to note that what crawling appeared 
involved only the uncut part of the body. 

It being established that the side stimulated produces the 
turning, the question may be raised, how, supposing in these 
longitudinally split individuals that this side does produce 
the reaction, is it known that it does this by lengthening 



GIO EAYMOND PEAEL. 

along its outer margin rather than by actively contracting on 
its inner cut margin ? This question may be answered by 
operative experiments of a different character. If the side 
stimulated, acting independently, produces the reaction by 
lengthening on its own outer side, then an isolated longitu- 
dinal half of the body ought to be able to give only one 
reaction wherever stimulated, or, in other words, it ought 
always to turn towards the same side. Furthermore, such a 
piece ought always to turn towards the cut edge, since onl}' 
on the side opposite to this has it a margin possessing 
the necessary circular muscles for extension (vide sup., 
pp. 556, 557). On the other hand, if the contrai'y view is 
correct, that the turning away is due to contraction of the 
longitudinal muscles on the side opposite that stimulated, 




Fig. 18.— Showing the appearance of a longitudinal half of a 
phinarian wlien at rest. 

then such an isolated longitudinal half of the body ought to 
be able to turn either way, according to the localisation of 
the stimulus, since there are longitudinal muscle-fibres along 
the cut edge as well as along the other. We may determine 
from experiments which of these two views is correct. 

Unfortunately, it is impossible to get any clear evidence 
on this point from entirely separated longitudinal halves of 
tiie Avorm. When a planarian is split in two lengthwise 
each of the pieces immediately becomes strongly contracted 
longitudinally on the cut side, the apparent purpose of this 
reaction being to reduce the exposed surface at once to a 
minimum. After this strong contraction has taken place, 
giving the piece the form shown in Fig. 18, no further 
pi'ogressive movement can take place, and the general tonus 



MOVEMENTS, ETC., OF FUl'^Sn-WATER PLANARIANS. Oil 

becomes immediately very much lowered. In view of these 
facts it is impossible to get any very trustworthy results 
from the stimulation of such a piece. 

There is another operation, however, which, while it does 
not isolate completely two longitudinal halves of the body, 
yet does separate into longitudinal halves the essential 
reacting parts, namely, the head regions. This is the 
splitting- ot' the worm in the middle line for a short distance 
back from the anterior end, as shown in Fig, 12. After this 
operation the two anterior pieces move about violently and 
independently for a time, taking all the various positions 
shown in Fig-, ]{). The animal soon recovers from the immc- 








FiG. 19. — Diagram sliowing tlie different positions taken by the two 
components resulting from longitudinal splitting of the liead. 

diate effects of the opei-ation, glides about in a normal way, 
only at a rather slow rate, and responds well to stimuli. The 
anterior piece keeps comparatively straight, there being much 
less tendency to contraction on the cut side than when the 
split extends the whole length of the body. The reactions of 
such a specimen to mechanical stimuli are as follows. To 
stimuli applied at the posterior end along the sides of the 
body the reactions are precisely the same as those already 
described for the normal individual. Stimulation in the 
regions a a (Fig. 20) of moderate or strong intensity produces 
the negative reaction. The organism turns away from the 



612 RAYMOND PEARL. 

side stimulated quite as promptly and in the same way as 
does a normal specimen. If now the cut edges A and B (Fig. 
21^) are stimulated in the same way (a needle may best be 
used for this) the specimen will always turn towards the 
stimulus. This can best be brought out by describing a 
typical case in which a series of fifty stimulations in the 
regions A and B were made on a favourable individual cut in 
this way. In thirty-nine of the reactions the animal turned 
towards the stimulated side. That is, if the stimulus was 
applied at A the animal turned in the direction of the arrow 
a ; while if B was the stimulated edge the reaction was in the 
direction of the arrow b. In eight of the remaining eleven 
trials the reaction was indifferent. The animal stopped at 




Fig. 20. — Operation diagram. See text. 

stimulation and then started moving straight ahead again, 
the stimulus evidently having been ineffective so far as 
special reaction is concerned. In only three cases out of 
fifty did the specimen turn away from the stimulus. Since 
it required the greatest cai-e in manipulation to give the 
stimulus to one cut edge without touching the other side, 
especially in view of the fact that the animal was moving all 
the time, it seems very probable that in these three cases a 
stimulus was accidentally given to the side wliich it was not 
intended to stimulate. The same general result of turning 

' After tills operation the two parts of the head usually take the position 
shown in this figin-e after the first spasmodic movements following the opera- 
tion have ceased. 



MOVEMENTS, ErO., OF FRESH-WATER PLANARIANP. 613 



towards the stimulus when applied to the cut edge was 
obtained in several other series with this same specimen, and 
many times with other specimens similarly mutilated. It 
will be seen that this is the result which would be expected 
if the turning away is due to lengthening of the side stimu- 
lated. Stimulation of either side of the cut portions, inner 
or outer, causes turning in the same direction, and that 

h 




L 




Fig. 21. — Diagram to show Uie reactions to ineclianical stimuli and 
tlieir niecliaiiisms in tlie case of a specimen in which tlie head 
has been split longitudinally. For further explanation see text. 

direction is the one in which turniug would be caused pro- 
vided each piece did actively lengthen on its outer side. 
There seems to be no reason whatever, if the turning away 
were due to contraction of the side opposite that stimulated, 
why the specimen should not turn away from stimuli applied 
to the cut inner edges. This it does not do. There seems 
to be no escape, then, from the conclusion that the turning 



614 EAYMOND PEARL. 

uway from the stimulus (negative reaction) is due to a 
lengtliening of the side stimuhated. 

It may possibly be objected to the last experiment that 
the impulse from a stimulation at, for example, B (Fig. 21) 
took the path indicated by the dotted line in that figure^ and 
caused a contraction on the left side of the body, so that 
really the observed turning was the result of a contraction on 
the side opposite that stimnlated. To this objection it may 
be answered that by stimulating different points along the 
edge B it is possible to cause the point about which the 
turn occurs as a pivot to be located anywhere along the line x y. 
It is very evident that contraction of muscles in the region N 
can have nothing whatever to do with turning of the right 
piece about the point x. So this objection is without force. 

As the process of regeneration of a cut longitudinal half of 
the body goes on^ the piece will straighten out from the 
curved form it takes after the cut is made, and it is conse- 
quently possible to obtain specimens in which the regenera- 
tion of the missing half of the body has produced only a very 
small amount of new tissue, and which are at the same time 
nearly straight in outline and able to make progressive move- 
ments. The reactions of such partially regenerated speci- 
mens are of importance as throwing light on the normal 
mechanism of the reactions. The reactions of a typical 
specimen of this soT't may be described in detail. On October 
10th, 1901, a small piece of the anterior end of a specimen of 
P. maculata was isolated. The piece was cut as nearly as 
possible in the form shown in Fig. 22, a. On October IGth 
the piece had the form shown iu Fig. 22, h. A narrow strip 
of new tissue had formed down the right side, and the forina- 
tion of the outline of the head and of the right eye was just 
beginning. At this time the reactions of the specimen were 
as follows. Stimuli applied at y caused the head to turn 
sharply away from the stimulus (typical negative reaction). 
This reaction was quite like that given by a normal individual 
stimulated in the same way. Stimulation at x, however, 
produced no trace whatever of a negative reaction. On 



MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 615 

stimu];ition at this point the speciinon contracted longitiuli- 
nally, and then staited moving ahead again in exactly the 
same direction in whicli it was going before stimulation. It 
was impossible to indncc any turning away following stimula- 
tion of the side ,r, althongh this was tried many times. 

Now it is evident that this specimen comes very near 
to being an isolated longitudinal half-planarian. All the 
structures of the original one half are present, and there 
is only a very little of the other side of the body produced in 
the line of new tissue, down the originally cut edge. In this 
new tissue there is probably very little differentiation, and the 
muscle layers are not well formed. It Avas brought out above 
(p. GIO) that an isolated half of the body ought to be able to 





Fig. 22. — o. Operation diagram, b. Piece wliich regenerated from A 
in Diagram a. The new tissue is indicated by stippling. 

give only one reaction, or, in other words, ought to be able to 
turn the body in only one direction in response to stimulation, 
provided this turning is due to an extension of the stimulated 
side. We find precisely this result in the regenerating speci- 
men just discussed. It turns away from stimuli applied at y 
because on that side are present all the muscles necessary for 
extension just as in a normal animal. It does not turn away 
from stimulation of the side x because it has not the necessary 
muscles for extension on that side. On the view that the 
turning away is due to contraction on the side opposite that 
stimulated, there is no reason why stimulation at .r should not 
cause the animal to turn away from the stimulus, because the 
opposite side ((/) has all its muscular mechanisms intact. 



616 RAYMOND PEARL. 

The reason why the specimen in this last expei'iment does 
not turn towards the stimulus when stimulated on the side x, 
is apparently because the regeneration has proceeded only far 
enough to produce just enough new tissue to form the 
beginning of a new side to the body. This new side receives 
the stimulus and is sufficiently potent to determine the re- 
action of the whole (the straight longitudinal contraction), 
but is lacking in the mechanism necessai-y to produce its own 
proper reaction, the negative reaction. On the other hand, 
in the case of the individual with the split anterior end, each 
piece turns towards the stimulus after stimulation of the cut 
edge because here only one half the organism is present 
either to be stimulated or to react; there is not even the 
beginning of the formation of a new side along the cut edge. 

Putting all the evidence together, I think it must be re- 
garded as demonstrated that the turning away from the 
stimulus in the negative reaction to mechanical stimuli is due 
to an extension of the side of the body stimulated. This 
extension is brought about by the contraction of the circular 
and dorso -ventral muscle-fibres — probably also assisted by 
the transverse and oblique systems of fibres — in the region 
stimulated. This reaction is a simple reflex act involving 
only the side stimulated. The normal organism, so far as this 
response is concerned, is to be considered as composed of two 
identical, but in a certain sense independent longitudinal 
halves. Thus, representing these halves diagrammatically, 
as in Pig. 23, a, the evidence presented indicates that stimula- 
tion of one side of the worm, as A, causes a reaction in that 
side, and, so far as essential features of the directive reactions 
go, only in that side. The movements of half A after its 
stimulation determine and, in fact, cause the reaction of the 
whole animal. Furthermore, these longitudinal halves retain 
their individuality as halves if they are isolated from each 
other. A separated half-worm (longitudinal) reacts as a 
half-worm, just as it did when in connection with the other 
half in the body, and not, as might perhaps be expected on 
a priori grounds, as a whole worm. It reacts as a whole 



MOVEMENTS, ET(!., OF FIlESH-WATEIi PLANAKIANS. 617 

worm only ufier a uuw half lias been regenerated along its 
cut edge. Tlie various stages in the change from the 
reactions as a half-worm to those as a whole worm can be 
followed step by step as regeneration proceeds. The new tissue 
formed along the cut edge very quickly takes on some of the 
functions of a side. Wiien only a narrow strip has been 
formed it serves for the reception of the stimulus, and hence 
stops the reaction of the opposite side, as in the experiment 
last discussed. To make the meaning more clear, reference 
may be made to diagrams h and c of Fig. 2o. In h is repre- 
sented, in a straightened position, the half B of a normal worm 




B 



A c 

Fig. 23. — Diagrams to show the relations of the halves of the body 
of Planaria to the reception of st.imuli, ant] the reactions 
tliereto. See account in text. (Tlie pharynx is omitted for 
the sake of clearness.) 

immediately after being separated from the other half, while 
c represents the same half after regeneration has begun and 
a strip of new tissue has been formed down the cut edge. 
Now stimulation of the cut edge of h causes the anterior end 
of the piece to turn towards the stimulus, i. c. to give its 
own proper negative reaction (cf. experiment given above on 
slitting anterior end). This is because in this case it is 
side B that is stimulated, although along its inner edge. 
Stimulation along the right-liand edge of c does not cause 
the turning towards the stimulus, because in order that this 



618 RAYMOND PKARL. 

may take place it would be necessary for the side B to give 
its proper negative reaction. It cannot do this because it is 
not directly stimulated, but the new very small side A is 
stimulated. This side may not have the necessary muscles to 
give a negative reaction itself — as in the experiment 
described above, — 3'et may receive the stimulus and so 
indirectly prevent B from reacting. Another way of ex- 
pressing this same fact is by saying that in regenerating 
longitudinal halves of planarians the physiological middle 
line remains at the line of the former cut edge for some time 
after regeneration has begun. ^ In connection Avith this 
discussion of the reactions of half-animals it is greatly to be 
regretted that Willey ('97) did not get any data on the 
reactions of the remarkable form Heteroplana. In this 
form we have a natural " half-planarian," or very nearly that. 
One side is so greatly atrophied as to be practically absent. 
It seems to me very probable that this organism would react 
to stimuli in much the same way that a longitudinally split 
specimen of Planaria, which had begun to regenerate, 
does. 

I do not wish it to be understood from the analysis of the 
negative reaction which has been given that I intend to 
maintain that in this reaction the side opposite that stimu- 
lated never contracts longitudinally. It probably often does 
this, especially in cases of very strong stimulation which cause 
a general excitation and reaction of the whole body. I have 
merely wished to show that the fundamental basis of the 
negative reaction is the extension of the side stimulated. It 
seems to me quite possible that it may be shown by close 
analysis in other cases that supposedly crossed reflexes are 
not fundamentally such at all. 

We may now pass to a brief consideration of the mechanism 
of the positive reaction of the planarian to mechanical 

' 1 liavc rc'coids in my notes of cxpcrinientb wliicli bliow tliat in llie case of 
oblique cuts the physiological middle line remains at the cut edge until after 
the new head is well formed in (he new tissue on tlie obli(|ue edge. Lack of 
fcpace forbids detailed desciipUon of these expeiiinenls here. 



MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. G19 

stimuli. As has been sLowu above^ removal of tlio anterior 
end of the body containing the brain causes the disappearance 
of this positive reaction, and this result is probably due rather 
to the lowering of tonus than to the removal of any special 
centre having the causation of this reaction as its function. 
Additional evidence on this view that lowering of the tonus 
is the chief cause of the disappearance of the reaction is found 
in the fact that other injuries to the head, such as longitudinal 
splitting, which produce a lowering of the general tonus, also 
cause the disappearance of the positive reaction. 

This very close dependence of the reaction on the general 
tonic conditions of the organism makes its analysis difficult, 
but it seems most probable that its mechanism is as follows : — 
a light stimulus, when the organism is in a certain definite 
tonic condition, sets off a reaction involving (1) an e(|ual 
bilateral contraction of the circular musculature, producing 
the extension of the body ; (2) a contraction of the longi- 
tudinal musculature of the side stimulated, })roducing the 
turning towards the stimulus (this the definitive part of the 
reaction); and (3) contraction of the dorsal longitudinal 
musculature, producing the raising of the anterior end. In 
this reaction the sides do not act independently, but there is 
a delicately balanced and finely co-ordinated reaction of the 
organism as a whole, depending for its existence on an 
entirely normal physiological condition. It is to be noted, 
however, that the definitive part of the reaction, namely, the 
turning, is a response of the side of the body stimulated. 
This point is one of fundamental importance for the general 
theory of the reactions. 

The mechanism of the other reactions to mechanical 
stimuli are evidently very simple. The crawling movement, 
which must be considered as the specific reaction to mechanical 
stimulation of the posterior region of the body, is due to 
rhythmical contraction of the longitudinal musculature. The 
only other reactions to mechanical stimulation are local con- 
tractions, whose mechanism is evident. 

e. Features in the General Uehaviour of the 



620 RAYMOND PEAKL. 

Organism which the Reactions to Mechanical 
Stimuli explain. — That much of the behaviour of pla- 
narians in their natural surroundings is the result of the re- 
actions above described is very evident to any one watching 
them. Among specific features of this sort in which these 
reactions play a part may be mentioned the escape from 
enemies or harmful surroundings, the getting of food (to be 
discussed in detail later), the localities chosen for coming to 
rest, the behaviour on meeting solid obstacles in the path of 
movement^ the passing on to the surface film, etc. All 
of these need not be discussed specifically, as their relations 
will be evident enough on a moment's thought, but the last 
two deserve special mention. 

The behaviour of planarians on meeting solid bodies in 
their path in the course of movement is entirely made up of 
reactions to mechanical stimuli. The behaviour in detail is 
as follows : — If a gliding specimen meets squarely head-on an 
obstruction of considerable size, so that it cannot glide over 
it without changing to some extent the position of its long 
axis, it will stop an instant^ raise the head, let it drop down 
till it touches the obstruction again, and then glide directly 
up on to and over the solid body. This behaviour is invari- 
able, so far as my observations go, if the worm meets the 
obstruction squarely. It is at once seen to be merely a 
special case of the usual reaction to a weak mechanical 
stimulus, characterised by the raising of the head. The 
behaviour is evidently purposeful in the long run, because it 
will take the organism up on to food material just as well as 
indifferent bodies. If the gliding worm meets the obstruc- 
tion obliquely the behaviour depends in large part on the 
physical nature of the object. If it is food material, or some- 
thing else of a rather soft and yielding texture — as, for 
example, another planarian, — the worm will immediately raise 
the head, turn it towards the object, and crawl up over it. 
This behaviour is evidently the typical positive reaction to a 
weak mechanical stimulus. A special and rather curious 
case of this positive i-eaction, which 1 have twice observed, 



MOVI'IMENTS, KIT'., OF FIfKSIT-WATKT? I'LAN AKIANS. C21 

appeared wlien two ?-periin(Mis o-liditiQ* alono-, wifli the nnterior 
ends slightly r:iised in tlu^ iioi'inal inaiiiicv, met liead-on. 
Both were siinnltaiieonsly stimulated to the positive reaction 
and raised the anterior ends, and then let them di'op ai>-ain. 
As they came down the two ventral surfaces were l)roni;ht 
squarely together in the way shown in Fig. 2t; then each 
started gliding up the ventral surface of the other. In a 
movement ns a result of the constantly changing form of the 
body, the ventral surfaces slipjiod off fi*om one another and 
the two worms went on their way. When the obstruction is 
a hard body, as a piece of glass, the specimen meeting it 
obliquely usually turns the head away slightly at the first 
contact (negative reaction), and then glides along parallel to 
the edge of the body for a distance. If it happens to touch 
it again with the side of the head, it frequently gives the 
negative reaction and turns away again. After the solid body 



Fig. 2i: — Side view of two plaiiariaiis starting: to glide up on Uie 
ventral surfaces of cacli other. 

has been touched several times, however, the positive reaction 
is usually given, and the worm passes at once up on to the 
solid body. This behaviour is shown in Fig. 2h. The precise 
form of the behaviour on meeting obliquely a solid body in 
the path varies considerably with the general physiological 
condition of the individual. In case it is much excited, the 
first touch will induce a strong negative reaction, and tlie 
individual will turn away and pass out of the neighbourhood. 
In the cases whei'e the final positive reaction is preceded by 
two or three negative ones, it would seem as if repetition of 
what must be an almost identical stimulus causes it to l)e- 
come in effect weaker. Leaving aside all variations in the 
exact character of the behaviour on meeting a solid, the 
important point to be brought out is that all this behaviour is 
based on the simple reactions to mechanical stimuli. The 

VOL. 46, PART 4. NEW SERIES. S S 



622 



I^AYMONII TKARL. 



exact behaviour in any o-jven case depends on several 
different factors. These are the position of the animal with 
reference to the obstruction, the physicnl nature of the 
obstruction^ and the physiological condition, whether of 
greater or less excitation. 

So, again, with reference to the habit of the animal of 
moving about on the surface film, a problem is presented. 
When a specimen, gliding up the side of a dish, touches its 
anterior end to the surface film at the point where the latter 
joins the glass, it immediately gives a characteristic positive 
reaction, precisely like that in response to any other weak 
mechanical stimulus. The head is raised and turned towards 
the side from Avhich the stimulus came_, and then dropped 




^ 6 

Fig. 25. — 1, 2, 3, 4, 5, and fi are successive stafifes in the reactions of 
PI an aria on meeting obliquely an obstacle in its path. Tlie hcav)^ 
straight line represents the obstacle. 

again. As a consequence of this reaction, the head end 
comes to rest on the under side of the surface film at a point 
some little distance out from the side of the dish. The 
ventral surface of the anterior end of the body flattens out on 
the surface film, and the animal glides out on to the film, 
following the direction determined by the reaction of the ante- 
rior end. Thus it is seen that the going on to the surface film is 
only a special case of a response to a weak mechanical stimulus, 
i. e. the positive reaction, the film itself acting as the stimulant. 
The leaving of the surface film and passing down the side of 
the dish is evidently also due to the same positive reaction. 



MOVEMENTS, ETC., OF FHESH-WATKR TLANAHIANS. 623 

There ai-e a nninbcr of other points in the general behaviour 
which are directly rehited to the reactions to nieclianical 
stimuli, wliich \Yill be taken up later in connection with the 
other reactions, 

/. Summary. — Before passino- on to a discussion of the 
next subject, it may be well to summarise briefly the chief 
findings with reference to the effect of mechanical stimuli on 
planarians. 

It has been shown that the planarian responds in a well- 
nigli perfect manner to the localisation and intensity' of 
meclianical stimuli. Tt turns awa}- from strong stimuli (in 
the long run harmful) applied to the side of the body ; turns 
towards weak stimuli (in the long run beneficial, almost never 
harmful) ; it crawls rapidly away from strong stimuli applied 
to the posterior end; backs and turns away from similar 
strong stinmli applied at the anterior end. 

It has been shown, further, that these reactions have all 
the characteristics of reflex actions, complex, it is true, but 
still reflexes. 

The mechanisms of the reactions to unilateral stimulation 
are unilateral, and lie in the side stimulated. 

Discussion of the implications of these results on 
mechanical stimulation, with reference to the psychology of 
the organism and the general theories regarding the reactions 
of oro-anisms to stimuli, is deferred till the results from other 
sorts of stimuli are in hand. 

II. Reactions to Food and Chemical Stimuli. 

Evidently one of the most important factors in the sum 
total of the activities of any aquatic organism is its reactions 
to chemical substances. Its ability to receive chemical 
stimuli and react to them must be of prime importance in its 
struggle for existence, for in its natural habitat such an 
aquatic organism must be almost constantly encountering 
different chemical substances. Some of these may be harm- 
ful and some beneficial, and it would seem that if a species is 



624 



RAYMOND PEAIJL. 



to survive, its individuals must have some sort of reaction 
whereby they may avoid the harmful and take advautnge of 
the beneficial. In the case of planarians, the reactions to 
chemicals seem to be of about equal importance with the re- 
actions to contact stimuli in the general activities. Since the 
reactions to food substances are a special case of the reactions 
to chemicals in general, they may be discussed first. 

a. Food Reactions — The nature of the things used as 
food by fresh-water planarians has been discussed already in 
the section on '^ Natural Histor}-," and hence need not detain 
us here. 

A typical case of the food reactions to a bit of crushed 




Fig. 2G. — Diaj^ram sliowiiif^ the successive stages in tlie normal food 
reaction of PI an aria. A represents a small bit of meat. 

mollusc may first be described, to serve as a basis for the 
account.^ If a piece of the body of Physa which has just 
been extracted from the shell and crushed between the points 
of a pair of forceps is placed in a small dish containing a 
number of active planarians, it will result from chance alone 
that some of the flat-worms must in course of time pass near 
the food material. For a very short time after the food has 

^ The food reactions of Planaria have been briefly described by Bardeen 
(:01,a). 



MOVEMENTS, ETC., OF EIJESlI-WATEl; I'LANAEIAMS. 625 

been placed iu the dish specimeus may pass very near it — 
within two or three luilhmetres — without being affected in 
any way. They simply glide straight by as if there were no 
food there. After a few minutes have passed, however, it 
will be found that a worm coming near the food is affected 
in a very characteristic manner. Its behaviour is as 
follows: — When within about three or four millimetres of 
the piece of meat (Fig. -0, o) it stops abruptl}', raises the 
head, and turns it towards the food (Fig. 2G, h). As the head 
is raised and turned the gliding is resumed, and the head 
being almost immediately lowered, the movement is directly 
towards the food. Thus fur the reaction is evidently 
precisely like the positive reaction to weak mechanical 
stimuli, and so we may speak of it as the positive reaction to 
food, the reaction being the same in the two cases, though 
the stimulus differs. ^Vhen the anterior end of the head 



Tig. 27. — Diagrammalic siiie view of Planaria to show tlie 
*' gripping " ol' a bit of fooil, A. 

touches the food it flattens down upon it, and, if the con- 
figuration is such as to nnike it possible, " grips " it 
(Fig. 26, c). The details of this " gripping " (shown in side 
view in Fig. 27) are as follows : — The anterior end closes 
down over the very edge of the piece of food, or over the 
whole piece provided it is small enough, and then 
apparently squeezes it by contraction of the longitudinal 
muscles on the ventral surface of the head, 'i'lie action is 
very characteristic, and evidently forms an integral part of 
the normal food reaction. Its probable function will be 
brought out Inter. While it is taking place the worm as a 
whole stops its progressive movement and remains quiet. 
After the " gripping" has continued for some time the worm 
starts gliding ahead up on to the food. It passes forward 
till the point where the opening for the extrusion of the 
pharynx is located is approximately over the place pre- 



626 KAYMOND PEARL. 

viously "g-i'ipped" (Fig. 26,(1). Then the pliai'ynx is ex- 
truded and feeding begins (Fig. 26, e). After ti time tlie 
worm voluntarily leaves the food and glides off over the 
bottom. 

Having described the typical case of a food reaction,, we 
may take up some of the more important variations from the 
tyjoe, and describe the various phases in the reaction in 
greater detail. 

Starting with the very beginning of the reaction, it may 
be said that the distance from the food at which any effect 
on the planarian is produced varies greatly, as is to be 
expected. This distance, of course, depends on the extent 
which the juices or chemicals of the food have diffused from 
it. When a piece of meat is first put into the water 
specimens will pass very close to it without being stimulated. 
In fact, if a specimen finds a piece of food within three or 
four minutes after it is put into the dish, it will usually have 
done so as a result of accidentally coming in contact with it. 
As has been brought out above, when a gliding worm 
touches anything of a rather jaeldiug texture, like food, it 
iuimediately gives the positive reaction and passes up over 
it. This plays an important ])art in the getting of food, 
because, as I have found in experiments, unless the food is 
crushed and pressed with forceps the juices diffuse rather 
slowly, and for some time specimens will not give the 
positive reaction unless they actually touch the food. On 
the other hand, after the food has been in the water for 
some time, so that diffusion has taken place, the distance at 
Avhich specimens may be affected becomes quite considerable. 
I have seen specimens gliding by a small piece of meat at 
a distance of Ig cm. from it give the positive reaction and 
turn towards it. At greater distances than this food is not 
effective, according to my observations. The distance from 
food at which a given specimen will give the positive 
reaction and go towards it depends also on the })hysiological 
condition of the individual. Specimens in a state of general 
excitation will, as I have frequently observed, go closely by 



move:men'1',s, etc., of fkesh-wa'ier planaiiians. 627 

a jDJece of food without tiiruiug towards it, while other 
specimens in a more normal condition will give the positive 
reaction some distance from it. 

After the first specimen has begun feeding on a piece of 
material the zone of influence of that piece becomes almost 
immediately widened appreciably. As the number of feed- 
ing specimens increases the area in the surrounding water 
which affects others becomes corres])ondingly greater. This 
phenomenon is very striking in many cases, as an illustration 
will indicate. Several pieces of crushed snail were put in a 
dish with a number of ])lanarians. In a short time a 
specimen in gliding about the dish had come near to one 
of these pieces, had given the positive reaction and begun 
feeding. At almost the same time another of the pieces of 
food had " attracted " another specimen. The other bits of 
food were quite similar in every way to these two, and lay in 
the dish not far from them. Yet at the end of fifteen 
minutes the two pieces by which the first two worms had 
been affected were completely covered with feeding 
specimens, Avhile the remaining pieces of food, with a single 
exception,' did not have a specimen on them. This increase 
in the effectiveness of the food as a stimulus must be due 
to the diffusion of more chemical substance from it. 
Apparently the increase is due either to some secretion of 
the feeding animals or to some change which they induce in 
the food. It is probably due to a combination of these two 
factors. That a digestive secretion is poured out through 
the pharynx of the feeding worm is well known, and clearly 
shown by the appearance of a piece of food on which a 
specimen has been feeding. The surface of the meat is 
turned white, and rendered very soft and almost flocculent. 
It is probable that this digestive secretion acts as a positive 
chemotactic stimulus to other worms, and that coupled with 
this there is an increased diffusion of juices from the food 
itself caused by the changes which it is undergoing. 

The reaction which is caused by this chemical stimulus 

' One piece ruiUicsl lemuved fruiii the ollicib luul ;i single specimen on it. 



628 KAYMOND PEARL 

fi'uin the food is evidently esseutially the same thiug' as the 
positive reaction given to weak uiecliauical stimuli. It con- 
sists in a turning of the anterior end of the body towards 
the source of the stimulus. There is no reason for supposing 
that its mechanism is in any way diiferent from that of tlie 
same reaction to mechanical stimuli, and hence this need not 
be further discussed here. A question of prime importance 
with regard to this positive reaction in response to chemical 
stimuli, Avhicli was not taken up before, is — how well localised, 
with reference to the stimulus, is the reaction ? or, in other 
words, how precisely does the anterior end point towards the 
source of the stimulus, — in this case food ? Have we here a 
clear-cut orienting response ? In answer to this problem it 
may be said that when the worm is only a short distance 
from the food the response is very precise. The anterior 
end is brought by the first positive reaction so as to point 
exactly towards the meat, and as the worm glides ahead it 
never misses it. This is true where the specimen is near 
enough (usually within three quarters of its own length), so 
that the stimulus which reaches it is a fairly strong one. 
In case the worm is stimulated near the edge of a large 
diffusion area when the stimulus is very weak, the first 
reaction may not suffice to direct the animal straight towards 
the food. In this case the behaviour is usually like that 
shown in Fig. 28, in which the line B, B, B, represents the 
effective margin of the diffusion area of the piece of food A. 
(By " effective nuirgin " is meant the line outside of which no 
effect is produced by the food on passing specimens.) The 
first reaction which the worm gives on reaching this diffu- 
sion area (Fig. 28, 1 and 2) is a weak positive one. It then 
proceeds on the new path into tliis area, but not directly 
towards the food. After a short time, however (Fig. 28, 3), 
it is again stimulated to a })ositive reaction" (4). This time 
both the stimulus and the reaction are stronger than before, 
and the worm is directed more nearly towards the centre of 
diilusion, but still not exactly. When it gets oj^posite the 
food again (5) another positive reaction (0) is given, and this 



aJOVEMENTS, ETC., UF FUESU-WATEU I'LANAlilANiS. 629 

time, since the stiuiulus is a rather slruug' one, the reaction 
is a very precise ouO; and the subsequent movement carries 
the animal directly io the food (Fig. 28, 7). This heluiviour 
is typical for this sort of stimulation, but may vary in its 
component phases, depeudiug- on the relative strengtli of the 
stimulus — the distance from the food when first stimulated. 




N B 



Fig. 28. — Diagram showiiij; llic rcaclioiis of Plaiiaiia l,u food (A) 
from which juices liavc l)i:cii diirusiiii^ into t ho walcr for some time. 
B, B, B, represent the ell'ectivc marj^in of the (liH'usiDii area of the 
food A. 1, 2, 3, i, 5, 6, and 7 are successive positions talvcn by 
the organism. 

Thus eithei" two or as many as four positive reactions niay be 
necessary to bring the animal to the food. This shows 
clearly that with reference to chemical stimuli, the precision 
of localisation of the positive reaction decreases as the in- 
tensity of the stimulus iliiuinishes. Indeed, I have observed 
what is L'viileutly nu unlocalised positive reaction, although 



030 



EAYI\IOND PEARL 



this seems piiradoxical. The behaviour was as follows : — A 
larfro diffusion area had beeu formed^ and a specimen was 
stimulated to a weak positive reaction at a distance of about 
twice its own length from the food (Fig. 29, 1). It passed 
into the diffusion area, but did not give another positive reac- 




/ B 



Fig. 29.— Showing the reaction of ii plauariiin to a very weak food 
stimulus. Letters as in l"ig. 28. 

tion when opposite the food, but instead glided by and away 
from it. When it had gone some distance in this direction 
it stopped and gave a very clear and characteristic positive 
reaction, so far as the form of the reaction indicated, but 
with the turn away from instead of lowai'ds the centre of 



MOVEMENTS, ETC., OF FUESH-WATI-:!; PI-ANA l.'IANH. 631 

cliifiisioii. There was no doubt of llie elianioter of the 
reaction ; the head was raised and the body turned in the 
usual manner of the positive reaction, whicli one can never 
mistake after once having- become familiar with it. The 
specimen kept on in the path determined by this last 
reaction (Fig. 29, 4), and passed entirely out of the region of 
the food. Evidently in this the Avorni was stimulated very 
weakly by a chemical, and the stimulus was nearly as strong 
on one side of the body as on the other, and when the reflex 
was set off it was on the wrong side of the body. This is 
not the usual result of weak stimulation, and has been 
observed in only two cases, but it serves very well to show 
the decrease of the power of localisation when the stimulus 
is very weak. 

When, as fi'equeutly ha])})ens, the worm approaches the 
food exactly head-on, the reaction usually consists merely 
of that portion of the reflex expressed in the raising of the 
head, while the worm keeps on in its straight path till it reaches 
the food. The head may be waved from side to side slightly, 
but the general direction of motion is not changed. The 
action evidently corresponds to the positive reaction following 
weak mechanical stimulation of the dorsal surface of the head 
in the middle line, as described above. In some cases, how- 
ever, I have observed very active and hungry specimens of 
Dendrococlum, sp., which were going straight towards the 
food, give a complete positive reaction and turn to one side 
and start off in a new direction away from the food. This, 
however, of course brought the specimen at once into a 
position where the stimulus was acting unilaterally, and it 
again gave a positive reaction, this time heading it again 
for the food, which it usually reached without further 
reaction, liut in some cases I have observed the specimen 
give so strong a reaction as to be taken almost directly 
away from the food by the subsequent movement, and, 
passing out of the area of diffusion, fail to reach it at all. 
Specimens behaving in this way Avere ''wild" in their 
general reactions. The responses were very vigorous, but 



632 IJAYMOND PEARL. 

not localised with refeieucc to tlio sLiiuulus with the utjiial 
precision. 

The ''gripping" of the food substance by the anterior part 
of the worm is a very characteristic feature of the normal 
food reaction. Its exact form depends on the configuration 
of the food or other body "gripped." In its most typical 
form, where the food material is in the form of a cylinder, or 
approximately such, the action reminds one of the action of 
the human hand in grasping a stick. The tip of the head 
closes over the material in the same way that the fingers do, 
while the region just behind the auricles bears the same 
relation as does the proximal part of the palm, just in front 
of the wrist, in grasping. After the head has been placed 
over the material in this way it can be seen to contract 
rather strongly, and thus literally squeeze the food. In case 
the surface contour of the food does not admit of this reflex 
being carried out in its typical form, as close an approxima- 
tion to this is made as possible. To compare again with the 
human hand, when the surface is flat, or forms the surface of 
a cylinder of large radius, the ventral surface of the head is 
pressed closely to it, the tip attempting to dip in, as it were, 
below the surface, in just the same way that a man "claws" 
with his finger tips in attempting to obtain a liold on a 
similarly configured body, too large for complete grasping. 

While the "gripping" is in general a very characteristic 
feature of the food reaction, it may be omitted in rather 
exceptional cases. The cause for the omission where it 
occurs, or any laws governing the matter, I have not been 
able to discover. A necessary accompaniment of the 
"gripping" of the food is the cessation of the forward 
movement ot" the animal as a whole. This pause when the 
food is first touched by the anterior end and before the worm 
passes up on to it, occurs in practically every case, whether 
the grijipmg accompanies it or not. The length of the pause 
is, of course, considerably greater when the " gi'ipping " 
occurs than when it is absent. 

The function of the "gripping" of the food material before 



MOVEMENTS, KTC, OF I'K'KSTT-WATKi; I'LANAinAXS. G33 

feeding begins is not inmicdiately apparent, but I am inelined 
to think its purpose is to intimately test the substance with 
regard to its availability as food. Some evidence on tliis 
point and further discussion regnrding it will be introduced 
later. 

After the preliminar}- pause nnd ^^ gripping" of the food 
the worm glides up on to it to begin active feeding, l^he 
position taken by the worm brings out a very nice correlation 
in reflexes. In a ver}' large number of cases (certainly over 
75 per cent., so far as my observations have gone) the worm 
advances over the food until the pharyngeal opening is 
exactly over the place where the first "gripping" occurred, 
and there the pharynx is extruded iind feeding begins. 




Fic. 30. — Diagram showing great extension of the pharynx. Tlie 
stippled area represents food substance on wliirh the phmarian 
is rest in?. 



When the worm reaches this position the posterior i^art of 
the body relaxes and takes on the appearance chai-acter- 
istic of the resting specimen. The pharynx is thrust out, 
and becomes attaclied very quick!}'. As it passes out through 
the opening in the body-wall it becomes usuall}' considerai)ly 
extended, and its diameter becomes correspondingly smaller 
than when it is in the pharyngeal sac. It ma}' or may not 
attach to the food directly beneath the body. When con- 
ditions are favourable it usually does, and consequently 
cannot be seen on looking down on the animal from above. 
On the other hand, I have frequently seen it stretched out 
and attached some little distance to one side of the body, as 
shown in Fig, 30. The stinnilus, causing the extrusion of the 



634 RAYMOND rFAKT,. 

pliarynXj is the contact of food or other solid body with tlie 
pharyngeal region of the ventral surface, together with an 
appropriate chemical stimulus. The pharynx is not extruded 
until the animal gets up on to the food so that the opening' of 
the pharyngeal sac is in direct contact with it. This can be 
demonstrated by direct observation by the use of a very 
small piece of food material and a plane mirror placed 
beneath the glass dish in which the specimen is moving. By 
lifting gently the posterior end of the body on a needle it can 
also be seen that the pharynx is not extruded before it is over 
the food. The most striking illustration of the correlation in 
the reaction which brings about the extrusion of the pharynx 
when it is just over the food, is to be seen when a specimen 
of the nemertean Stichosterama asensoriatum is used as 
food, and the long axis of the planarian and of the nemertean 
are at right angles to each other. After first " gripping " 



Fig. 31. — Diagrammatic Icngiludinal sccHon of a planarian feeding 
on a nemertean (shown in cross-section at x ). 

the nemertean the planarian glides along over it until the 
pharyngeal opening is just above it, and then pauses, and the 
pharynx is extruded and attached (« and h, Fig. 31). These 
facts strongly indicate that the effective stimulus for pharyn- 
geal extrusion is received, at least in part, in the pharyn- 
geal region itself. That it is necessary for both contact 
and chemical stimuli to act to produce the extrusion of 
the phaiynx may be shown by experiments on specimens 
gliding on the surface film ventral side uppermost. If, with 
such a specimen, a chemical known to produce under other 
conditions extrusion of the pharynx, is allowed to come in 
contact with the pharyngeal region, there is no result. Of 
course in performing this experiment proper precautions 
were taken not to disturb the animal by allowing the solution 
to drop upon it. Another demonstration of the same fact that 
a chemical stimulus alone does not suffice to cause extrusion 



MOVEMENTS, V.TC, OF FltRSTT-WATRK PLANARTANS. 635 

of tlio pharynx is that specimens immersed in favonvahle 
solutions, such as sn^^ar solutions, do not show tliis phe- 
nomenon. 'IMiat mechanical stimulation alone does not suffice 
is demonstrated by the fact that planarians pass over and rest 
on other planarians without extruding- the phai-jnx, althoug'h 
the consistency of their l)odies is evident!}' much the same as 
that of the animals used as food. In fact, they will l)e 
used as food frequently if they ai"e wounded so as to afford 
the proper cliemicfd stimulus. The stimulation of tlie anterior 
end of the body by the food seems also to be necessai"y before 
pharyngeal extrusion takes place. The data on this point will 
be presented later in connection with operation experiments. 

The appearance of the body on the food is quite charac- 
teristic. As mentioned above, when the pharynx is extruded 
forward, movement stops, and the posterior part of the body 
becomes more or less relaxed. The anterior third of the 
body, however, keeps in movement during a considerable 
part of the time the specimen is feeding. The head is waved 
about from side to side, and touched to the food or the 
bottom of the dish here and there. It keeps its character- 
istic extended form to a greater or less degree. A favourite 
position is for the anterior third or half of the body to lie on 
the bottom and move about, while the posterior part lies up 
on the food. This is the position most frequently seen in 
specimens feeding on a rather small piece of meat. Wiieu 
the anterior end gets on the bottom it gives every appear- 
ance, in many cases, of attempting to glide away, and being 
only restrained by the attachment of the pharynx to the 
food. In other cases, however, the anterior end remains 
quiet. The importance of the attempted movement will be 
brought out later. As has been mentioned above, the flat- 
worm is able to move off and drag the food still attached to 
the extruded pharynx along behind it. In the fastening of 
tiie food to the body in this case the sticky slime undoubtedly 
assists the pharynx. 

After the food has been softened by the digestive juices, 
it is taken into the body through the pharynx. 



636 T^AYMOND TEARL. 

After the worm Ims been feeding for n, certnin leno-tli of 
time it will detach the phaiynx and spontaneously move off 
from the food, the pharynx: being withdrawn again into its 
sac. The length of time after the beginning of the feeding 
at which this takes place varies very greatly in different 
cases. I have observed a specimen which fed on a piece of 
mollusc for as long as an hour and thirty minutes, while in 
other cases the worm may stay on the food only ten minutes, 
or even less. Judging from the rate at which food is taken 
np while the animal is feeding during the day, and from the 
fact that pieces of meat left in the dish overnight are almost 
entirely consumed by morning, it would appear that much of 
the time during the night is spent in feeding when any 
material available for the purpose is at hand. While the 
anterior end of the feeding worm retains its normal sensi- 
tiveness to stimuli, it nevertheless requires considerable 
stimulation to induce a feeding worm to leave the food. 
Shaking of the dish, which would ordinarily set all resting 
specimens into rapid movement, has little or no effect on 
feeding specimens. If a worm is suddenly pulled off a piece 
of meat on which it is feeding a very good view of the 
extruded pharynx may usually be had, as this organ is 
retracted somewhat slowly when torn from food in this way. 

So far as I have been able to discover, the presence of 
food in the immediate neighbourhood of a resting planarian 
has no effect upon it. Apparently the stimulus afforded by 
crushed meat is not sufficiently strong to produce a response 
from such an individual. The following experiment copied 
from my notes will show this. 

May 14th, 1901, 3.10 p.m. — A piece of freshly crushed 
snail was placed 1 mm. distant from the anterior end of a 
resting specimen. No reaction or other effect produced. 

3.30 p.m. — Worm in same position as before. 

4.5 p.m. — No change. (At this time the worm was acci- 
dentally started into movement and the experiment conse- 
quently ended.) 

This lack of effect of food on resting specimens may be 



MOVEMENTS, ETC., OF FRESH-WATET^ PT-ANAIHANS. C37 

tlio reason for the statement of Bnrdeen (loc. cit., p 522) '' that 
worms which liaJ been kept in pure rain water for a week or 
two, and were thus in a hungry condition, would remain 
unmoved by the presence close by their side of a piece of 
fresh snail, a food much prized by them," 

1 . Food Reactions of Specimens after Opera- 
tions. — For the purpose of throwing" light on the general 
mechanism of the food reaction, experiments were tried on 
specimens cut in different ways. It is unfortunately very 
different from practical reasons to get many certain results 
from these experiments. Many of the results are negative, 
and hence not entirely conclusive. Since, however, some 
important facts have been brought out by these experiments, 
they will be described. 

The first operation which will be discussed is that of 
cutting the animal in two transversely. If such a cut is 
made in the region in front of the pharynx, the anterior 
resulting piece, after it has recovered somewhat from the 
shock effect of the operation, will show the following reac- 
tion. On coming into the zone of diffusion about a piece of 
meat it gives the positive reaction just as a normal worm 
does, and turns towards the food. On reaching the edge of 
the meat its behaviour is again like that of the normal 
animal ; it stops, usually " grips " the food, and then passes 
on over it. At this point appears the striking difference 
between the behaviour of this anterior piece, which, it must 
be remembered, has no pharynx, and the behaviour of the 
entire worm. The anterior piece after gripping the food 
glides up over it, and without the slightest change, even in 
the rate of gliding, passes down off of it on the other side. 
There is not the slightest indication of any stopping for the 
pharynx to be extruded. 

If the transverse cut is made farther back, so that the 
pharynx is included in the anterior piece, this will then 
behave with reference to food quite as a normal animal does. 
It will stop on the food and extrude the pharynx. 

The posterior pieces resulting from transverse cuts do not 

VOL. 40, PART 4. NEW 3EKIES. TT 



638 RAYMOND TEAEL. 

give any definite food reaction, so far as I have been able to 
ascertain, until they have been regenerated to some con- 
siderable extent. Posterior pieces from which only the head 
has been cut will glide by pieces of snail on which other 
wornas are feeding, without giving the slightest reaction.^ 
In experiments so arranged that the gliding posterior piece 
would just touch with its anterior end the edge of a piece of 
food, it gave no reaction. This same arrangement -with a 
normal w^orm practically never fails to call forth the positive 
reaction and bring the worm up on to the food. Posterior 
pieces placed gently on pieces of food material do not extrude 
the pharynx and start feeding, but immediately glide down 
from it and over the bottom of the dish. These experiments 
with posterior pieces have been tried many times and under 
varied conditions, in the hope that some sort of positive 
results might be obtained, but never with success. This is 
true for three days after the operation. After a new head 
has been fairly well formed the animal will react to food 
again. The behaviour of one of these posterior pieces on 
touching with the anterior end a piece of food is very 
strikingly different from that of a normal animal. The cut 
piece, if it touches with the front or sides of the anterior end 
the smallest shred of food material, or any other substance, 
gives a well-marked negative reaction, and goes in a new 
direction away from the obstruction. It does not, as a rule, 
crawl up over anything which it meets squarely " head-on," 
but instead turns away. 

Thinking that possibly the pharynx might play a more or 
loss independent part in the normal food reaction, i. e., that 
it might have a set of reflexes of its own, not determined by 
the rest of the body, I tried experiments with the isolated 
pharynx removed entire from the body. Such an isolated 
pharynx will remain alive for a considerable period, and 
respond to stimulation. When first removed from the body 

' Bardeen (: 01, a) has sliowii that, if the transverse cut is in the region in 
front of the eyes the posterior piece (comprising in this case nearly the whole 
worm) will react normally to food. 



MOVEMENTS, KTf'., OE FIJESTT-WATER PLANA IITANS. 039 

it contracts I'lijtliinically in a longitndinnl direction for n 
time, and then comes to rest at about its normal Icuotli wlien 
in the body. Mechanical stimnlation causes merely lono'i- 
tudinal contraction, Avhile the presence of food near it has no 
effect whatever. Freslily crushed snail meat placed Avithin a 
millimetre of such an isolated pharynx had no effect upon it 
in the course of an honr. 1 have tried layino- the isolated 
pharynx directly on pieces of meat to see if there would be 
any tendency for the end of the orc^an to attach itself as it 
normally does. This was not done, nor was any other 
definite reaction produced. 

These operation experiments show, so far as they pfo, that — 

(1) The presence of the pharynx in the body (i.e., the 
functional ability to take food) has nothing to do with deter- 
mining- the reaction of the anterior end of the bod}^ to food 
stimuli. The anterior part of the body gives the same re- 
action to food in every case, without regard to whether so 
doing actually puts the animal in a position to get food oi' 
not. The i*eaction is only purposive under certain circum- 
stances; when changed conditions make it no longer purpo- 
sive, no adaptive change in the behaviour of the anterior end 
occurs. This shows clearly how little basis there is for con- 
sidering the behaviour towards food as anything of the 
nature of intelligent behaviour. 

(2) The stopping of the worm on the food under normal 
circumstances is due to the posterior half of the body, not the 
anterior. The behaviour of the anterior cut piece in gliding 
directly over the food is what one might be led to expect from 
the behaviour of the same part of the body under normal 
circumstances. As described above^ it was seen that the 
anterior end of the normal individual gives every appearance 
of attempting to continue moving forward while the posterior 
part is feeding, and is only prevented from doing this by the 
mechanical hindrance of the attached pharynx. In a sense, 
we may consider that in a large degree the work of tlie 
anterior end of the body with reference to feeding is over 
when it gets the animal up on to the food. 



640 RAYMOND PEARL. 

(3) The reception of the food stimulus is a function of tlie 
head. In other words, the head is the only part of the body 
capable of receiving very Aveak chemical stimuli. 

(4) Decapitated specimens do not extrude the pharynx, so 
far as my observations go, even though the proper normal 
stimuli are given the pharyngeal region. Presumably the 
brain is the necessary organ in this connection, as we have 
already seen that the sense organs concerned with the act of ex- 
trusion are not those of the head, but of the pharyngeal region. 

Bardeen (: 01, a, p. 178) states that '^ the simple reflexes of 
extending the pharynx and of swallowing are preserved after 
removal of the head. I found, by repeated trials, that one of 
the headless pieces could usually be made to eat if it was 
placed on its back on a slide in a small drop of water. Under 
the conditions mentioned the pharynx is usually protruded, 
and will engulf bits of food placed in the mouth." Regard- 
ing this conclusion, I can only say that in a large number of 
experiments with decapitated specimens I have never been 
able to induce extrusion of the pharynx, under conditions 
approximating as closely as possible to the normal. I do not 
wish to affirm that the decapitated planarian cannot extrude 
the pharynx, but merely that it does not when placed in 
situations which normally produce pharynx extrusion. 

(5) The pharynx is not an independent organ in its reactions, 
since, when separated from the body, it does not react with 
reference to the localisation of the stimulus, as it does when 
normally connected with the remainder of the body.^ 

2. Summary of Food Eeactions. — It is shown above 
that planarians have a very definite and characteristic set of 
reactions to food substances which enable them to become 
aware of the presence of food, and find it. The importance 
of these reactions in the life of the individual can hardly be 
over-estimated. While planarians, like many other lower 
organisms, can live for a considerable time without food, yet 
in the long run they must, of course, have it. The question 

^ Evidence on this latter point will be brought forward in connection with 
the reaction to chemicals. 



MOVEMENTS, KTC, UV FRESH-WATI'iR PLANAiaANS. 641 

of how a lower orguiiiisiu gvia its food, taking' advantage of 
the good and rejecting the bad, and thus apparently choosing 
one thing from several, is one of the most interesting and 
important in comparative psychology. 

The food reaction of planarians consists of an extremely 
well co-ordinated set of reflexes, which may be set into action 
by stimuli of two sorts, — first, chemical ; and second, 
mechanical. Both sorts of stimuli are, of course, given by 
the food. The first and most important of till the reflexes in 
the food reaction is the turning of the head towards the 
source of stimulation, followed by movement in that direction. 
This is the reaction which enables the animal to find food. 
Evidently it is the same thing exactly as what has been 
described as the positive reaction to mechanical stimuli; or, 
in other words, the positive reaction to mechanical stimuli is 
only a special case of the general food reaction. Its primary 
function is evidently the getting of food, whatever the stimulus 
which calls it forth. The reason for a food response following 
mechanical stimulation is to be found in the fact that it most 
frequently happens that many things (e. g., whole animals) 
which are available for food are not emitting chemical sub- 
stances into the water in sufficient quantity to cause an 
effective stimulus. If the planarian did not give a positive 
reaction after contact with such bodies they would be missed, 
and no advantage taken of them as food. By reacting 
positively to Aveak mechanical stimuli the animal is in a 
position to take advantage of the presence of food of all sorts, 
whether it is in condition such as to diffuse chemical sub- 
stances through the water or not. This fact that the animals 
react to food substances as a result of mechanical stimulation 
affords a possible explanation of the "gripping " phase of the 
general response. The purpose of this " gripping " may be 
to bring the sense organs of the head, which are sensitive to 
chemical stimuli, into very close contact with the substance 
in order to determine whether it possesses the chemical 
characteristics of food. In other words, this reaction is a 
"tasting" reaction, which is made necessary by the fact that 



642 RAYMOND PEAKL. 

the organism turus toward all bodies of a certain physical 
texture under most circumstances. Tlie active squeezing of 
the material in the "gripping" undoubtedly helps to press 
out to the surface any juices which may be in the material. 

In closing the section on food reactions it may be well to 
give a sort of general picture of the whole behaviour of 
fresh-Avater planarians towards food. The method by which 
the planarian finds material suitable for food is as follows : 

1. Chemical substances diffusing from food come in con- 
tact with the sensitive head region of the planarian ; or — 

The movino- animal touches with the head some soft sub- 
stance, and as a result of either of these two sorts of 
stimulation — 

2. The organism gives a positive reaction, i. e. turns 
towards the source of the stimulus. This reaction is very 
precisely localised in most cases, and is the most essential 
part of the whole food behaviour. Its mechanism has been 
previously described (v. sup., p. 619). 

3. When the anterior end squarely touches the food as a 
result of this reaction it typically closes tightly over it, 
giving what I have called the ''gripping'^ reaction. This 
reaction is evidently a very much specialised feeling move- 
ment for the purpose of closely testing the chemical nature 
of material. It is produced by a contraction of the ventral 
longitudinal muscles of the head region. While it is taking 
place progressive motion ceases. 

4. After this pause the worm glides over the piece of food 
till the opening of the pharyngeal sac lies over or nearly 
over the place " gripped/' and there the posterior part stops 
and the pharynx is extruded and attached to the food. The 
factors determining the place where the pharynx shall be 
extruded are (a) the stimulation of the ventral surface of 
the body in the pharyngeal region of the food (pure reflex 
factor), and [h) the presence of the brain, which probably 
acts as a co-ordinating centre for this reaction. 

•5. A digestive Huid is poured out through the pharynx, 
and the food is partly digested before being taken up. 



MOVEMENTS, ETC., OF FRESH-WATEK PLANARIANS. 643 

6. The softened food is taken into the body through the 
phaiynx. 

7. The animal spontaneously stops feeding after a certain 
time. 

The question now arises, if the normal process of getting 
food is at bottom in the majority of cases a reaction to a 
chemical stimulus, what is the uatui*e of the chemical sub- 
stance causing it ? Can the same response be induced by 
the use of different inorganic and organic chemicals ? Is 
there any relation between chemical composition and the 
intensity or form of the reaction ? To answer these and a 
number of other questions arising out of them recourse must 
be had to experiments in Avhich the nuture and concentra- 
tion of the chemicals affecting the organisms may be con- 
trolled. All the experiments of this kind I will group 
together under the heading — 

h. Keactions to Chemical ^Stimuli — Che mot axis. 

1. Reactions to Localised Chemical Stimuli. — 
This particular phase of the general subject of the effects of 
chemicals may be considered first, since it is most closely 
related to what has preceded on the food reactions. The 
plan of the experiments was to try the effect of a series of 
substances when applied to restricted areas of the body. A 
sufficiently large number of chemicals were used to include 
representatives from each of the main groups of substances 
which have been found to have marked effects on organisms. 

a. Methods. — The method Avhich was found to give the 
most satisfactory results in the application of localised 
chemical stimuli was the use of a capillary tube filled with 
the solution whose effects it was desired to test. The form 
of the tube used is shown in Fig. 32. The tubes were 10 to 
15 cm. long, and were made from glass tubing of about 
2'5 mm. internal diameter. Each end was drawn to capillary 
fineness, and then broken off so as to give an opening of the 
desired size. The opening at the upper end was made 



644 RAYMOND PEARL. 

slightly larger than that at the lower, which was used in 
giving the stiniulas. The tube was filled with solution by 
suction. The rate of diffusion can be regulated by changing 
the sizes of the openings, and can be determined for each 
tube from the rate at which the fluid sinks at the upper end 
of the tube. Considerable experimenting is necessary in 
order to get the best rate of diffusion for work on planarians. 
Since the animal is moving rather rapidly while the stimulus 
is being applied it is necessary to have reasonably rapid 
diffusion or the worm will not react at all, or not for so long 
a time after the stimulation has begun that one cannot be 
certain of the results. It is easily possible to get the 
capillary so fine that no results can be obtained. On the 
other hand, when it is too large the solution affects too large 
a portion of the body at one time, and furthermore, as will 
be shown later, may cause a rheotactic reaction of the 
organism. This, of course, introduces a possible source of 



Fig. 32.— Glass tube used in giving localised chemical stimuli. 

serious error. It can be avoided by frequent and proper 
control experiments. 

It will be well to describe in advance the conduct of a 
typical experiment and the precautions taken, so that it may 
not be necessary to repeat these details in the account of 
each experiment. Six to ten normal active planarians were 
taken from the aquarium dish and put in a Petri dish of 
about 10 cm. diameter, in freshly drawn, filtered tap water. 
Enough water was put in the dish to give a depth of about 
1 cm. Two or three of the capillary tubes with different 
sized openings were filled with the tost solution. These 
tubes were all tested before a final experimental series was 
begun, and usually only one which had been found to allow 
diffusion at the satisfactory rate was used. In some cases, 
however, varying degrees of sensitiveness among the 
different specimens made it necessary to use for some in- 



MOVEMENTS, ETC., OF FRESH-WATER PEANARIANS. 645 

dividuals capillaries of faster or slower rates than what may 
be called the standard. After preliminary experiments to 
determine the relative sensitiveness of the different parts of 
the body to chemicals, attention was devoted almost entirely 
to stimulation of the head region, and consequently in tlie 
experiments which will be reported first the stimulus was 
applied only to the head, unless otherwise stated. The 
method of applying the stimulus was to place the ])oint of 
the capillary tube a short distance (about 2 mm.) from the 
place on the body to be stimulated. The animal was stimu- 
lated as it was gliding along in the normal way, and hence 
it was necessary to move the capillary tube at the same rate 
the animal moved in order to keep it opposite the same point 
in case the reaction was not given at the instant the capillary 
was put into place, which, of course, almost never happens. 
With a little practice one can move the tube along as the 
worm glides so as to keep the relative position of the two 
almost identically the same. Just as soon as a reaction had 
been obtained with a given specimen the capillary tube was 
removed from the water, so as to permit as little as possible 
of the chemical to get into the water surrounding the 
organism. After a series with any substance, the worms 
were transferred at once to a dish of fresh water before 
beginning another series. Further, in any long series, when 
for any reason it might be supposed that the water was 
becoming contaminated with the chemical to an extent 
sufficient to affect the results, the worms were transferred to 
another dish of fresh water. All through the course of an 
experiment frequent control tests were made by trying the 
effect on the worms of the water surrounding thein when 
diffusing out from the same tube used previously for the 
chemical. After each experiment the tubes were thoroughly 
rinsed by drawing distilled water back and forth through 
them many times. The tubes were also frequently discarded 
and new ones substituted. 



646 RAYMOND PEARL. 

The following substances were used in the experiments 

/-Nitric 
Mineral acids . \ Hydrochloric 



ISu 



Iphuric 
Oxalic 



Organic acids . ^ Citric 



Alkalies 

Salts of heavy metals 

Other salts . 



Cane-sugar. 



I Formic 
fSodium hydrate 
(Sodium carbonate 
fCopper sulphate 
(Zinc sulphate 
Sodium chloride 
Sodium bromide 
Potassium chloride 
Masfuesium chloride 



Distilled water. 

Since distilled water was found to have a decided effect in 
producing a reaction^ the solutions were prepared iu both 
distilled water and in filtered tap water. In case of any 
doubt; as with very dilute solutions, the effects of the solu- 
tions prepared in each sort of water were tested and 
compared. 

Since only qualitative results were desired, and for the 
practical reason of greater convenience; perceutage rather 
than molecular solutions were used. 

/3. Results. — The results are, in a way, so remarkable that 
they will be presented in some detail. 

Mineral Acids. 

Nitric (sp. gr. 1*42), i per cent. — This solution causes 
strong negative reaction. If applied to the head region the 
animal turns away from the side stimulated immediately; and 
strongly. If the stimulus is long continued the animal 
writhes and twists about violently. 

Stimulation of the posterior region causes the part where 



MOVEMENTS, ETC., OF FRESn-AVATEIi PLANA RIANS. 647 

the solution strikes to contract very violently, and the whole 
animal to start crawling ahead rapidly. This concentration 
is very injurious, and if its action is continued^ quickly kills 
the individual. It will be noted that its effects are the same 
essentially as those of strong mechanical stimuli applied to 
the same parts of the body. 

y*y per cent, and ^^^ per cent. — Kesnlts the same as in 4 
per cent. The animal is not as quickly and extensively 
injured by these solutions as by the former. It is to be 
noted that with these comparatively strong solutions the 
reaction time after stimulation of the posterior end of the 
body is so slow that this part of the body is permanently 
injured or destroyed before the animal gets away. 

-^^ per cent. — In some cases a well-marked positive re- 
action was caused by stimulation of the head region with 
this solution. The head would turn towards the mouth 
of the pipette in the characteristic fashion of the food reaction, 
or the reaction to weak mechanical stimuli. In other in- 
dividuals the reaction given was weakly negative, while still 
other specimens were indifferent. In cases where there was 
an indifferent reaction there was a local contraction of the 
side of the head stimulated. 

g*g- per cent. — Clearly marked positive reaction in large 
majority of cases after the stimulus has acted for some 
time. This solution never caused the negative reaction. Some 
individuals were, in a few cases, indifferent to this solution. 
This solution is too weak to start a resting specimen into 
movement. 

Y^y- per cunt, and weaker. — Indifferent reactions or weak 
positive. 

This acid appears to be a very strong stimulus for the 
negative reaction in concentrations down to j,, P<-'i" cent., 
while below that it is a rather ineffective stimulus, and the 
reaction when induced is positive. 

Hydrochloric, j\j per cent. — Strong negative reaction. 
There is nuticuablc in some cases a tendency for some 
individuals to turn very slightly towards the source of 



648 RAYMOND PKARL. 

stimulation before giving' tlie strong negative reaction. 
Stimulation of the anterior end of a decapitated specimen 
caused a slow negative reaction with long reaction time. 
This solution causes the change from the glide to the crawl 
when applied to the posterior end of a normal worm. 

yV per cent. — Negative reaction; rather weaker than with 
preceding solution. With this solution one specimen would 
turn towards the source of the stimulus until the head came 
into the strong acid near the mouth of the pipette, and then 
give the sharp negative reaction. 

:f J per cent. — Specimen A gave positive reaction in every 
case ; specimen B in about 50 per cent, of all cases, while 
iu the remainder of trials gave weak negative. Other 
specimens negative reaction. 

^L per cent. — Specimen A as in preceding case. Specimen 
B gave positive reaction in about 90 per cent, of all trials. 
Other specimens weakly negative reactions. 

Y^jj per cent. — All specimens give well-marked positive 
reaction. They glide up to the end of the capillary and 
"grip" it with the anterior end as in the food reaction. 
After holding on for a moment they let go and give a sharp 
negative reaction, indicating that the stimulus is still too 
strong when continued. This behaviour will indicate the 
machine-like character of the positive reaction. 

■^Q per cent. — In the majority of cases indifferent re- 
action. Remainder positive. 

To give an idea of the dependence of the reactions to 
chemicals on the physiological condition of tlic organism, the 
following series of experiments with HCl in solutions of y-^^^ 
per cent, and weaker concentrations may be described. It is 
to be understood that these experiments were carried out 
on different animals from those just given. 

y-g^ij per cent. — No sharp positive reaction. Specimens 
Avill give a weak negative reaction if the opening of the 
capillary is held very near the head. In most cases reactions 
are indifferent. 

^TU" P^^* cent. — One specimen gives positive reaction and 



MOVEMENTS, ETC., OF FRESIT-WATEK PLANARTANS. 649 

jrroos tlirono'li whole food rcnction on tlio end of tube. 
The remainder still give weak negative reactions. 

■cTU P^^" cent. — Reactions essentially the same as in ^r— ^ 
per cent. 

At this point this series was discontinued. It shows that 
any absolnte concentration for a chemical solution which will 
cause all planarians to give the positive reaction cannot be 
assigned. How a given individual will react to a given 
concentration of chemical depends almost, if not quite as 
much, on the individual as it does upon the solution. 

Sulphuric, j\j- per cent, and -^ per cent. — Caused imme- 
diate and violent reaction. Decapitated worm reacts like 
normal. This is evidently a very strong stimulus. 

jij per cent. — Caused strong negative reaction in majority 
of cases. One specimen reacted as follows : — the capillary 
tube being held some distance away from the head, it 
first gave a well-marked positive reaction. On coming 
into the stronger solution near the mouth of the tube it 
began strong convulsive contractions (evidently on account 
of too sti'ong stimulation). Ifc remained, however, at the 
same spot, and after a few minutes extruded the pharynx 
and swept it about over the bottom. The specimen re- 
mained this way for some time. The tube was, of course, 
removed immediately after the first positive reaction was 
given. A decapitated specimen in one case gave a very 
distinct positive reaction to this solution, the tube being 
held some distance away from the specimen. 

■giy per cent. — Negative reaction. Decapitated specimen 
gave positive reaction once. Tliis solution, applied to 
the posterior end of the body, induces the crawling move- 
ment. 

Too" P^'^' cent. — Negative reaction. Isolated pharynx con- 
tracts into a ball when stimulated with this solution. 

■j^jj per cent. — Positive reaction in one case. Remainder 
negative. Same result with pharynx as in ji^ per cent. 

■^^j; per cent., y'^t^tt pcr cent., and ttttto pci' cent. — With 
these solutions the reactions were for the most part negative. 



650 EAYMOND PEARL. 

In a few cases positive responses were produced^ but not 
regu]ixv]y. 

otVtt psi' cent. — Positive reaction in all cases. The 
whole food response was produced in case the end of the 
tube was left in position. The worms '^gripped" it, glided 
up on to it, and extruded the pharynx, in many cases running" 
the latter up into the lumen of the tube. Anterior piece, 
resulting from cutting animal in two transversel}^, acts like 
whole worm (positive reaction), but less strongly. Decapitated 
worm gave no response. In order to make sure that in this 
case it was the extremely diluted acid which was producing 
the result, numerous controls with distilled water and culture 
water and fresh tap water were tried on the same speci- 
mens, in alternation with trials with the acid. With tap 
water and culture water the specimens were indifferent ; 
but with the acid solution (-g-yVo P®^" cent.) mixed in either 
tap water or distilled water they gave a well-marked positive 
reaction. This showed clearly that the results were due to 
the acid. 

Summary. — With the three mineral acids tested it was 
found that to concentrations above a certain point the speci- 
mens always gave the negative reaction, while to concentra- 
tions below this point the positive reaction was given. The 
absolute value of this " critical point " varies widely with 
different individuals. The behaviour is essentially the 
same as that in response to mechanical stimulation, viz. to 
strong stimuli the negative reaction is given, to weak the 
positive. 

Organic Acids. 

Oxalic, i per cent, and yL per cent. — Sharp negative 
reaction. This solution affords a very strong stimulus 
and quickly kills the specimen. The negative reaction 
is very violent when once induced, but several speciuiens 
were killed before they turned away. There was notice- 
able a slight tendenc}^ to turn towards the stimulus the 
instant it was perceived, and before this could be replaced by 



MOVEMKNTS, ETC., OF FRESH-WATKIJ PLANAT^IANS. 651 

tlie negative reaction the specimens were nearly or quite 
killed. 

-^■^ per cent. — Convulsive negative reaction in tlie great 
majority of cases. In one case stimulation was followed 
by sharp positive reaction, succeeded by extrusion of the 
pharynx. 

-^jy per cent, and J^ per cent. — A few specimens on some 
trials give positive reaction, and then go into convulsive 
twisting movements as they get into stronger solution. 
Remainder negative. 

T^TT P^^' cent, and ^i- per cent. — Positive and weak nega- 
tive reactions about equally divided. 

UTir P^^' cent, and y^rg-Tv P^i' cent. — Positive reactions 
becoming proportionately more numerous. Negative re- 
actions are very weak when given in response to these 
solutions. In the cases where there is a positive reaction 
the full response is not given; the specimens go up to 
the mouth of the tube, but do not grip it nor extrude the 
pharynx. 

Y5Vn P^^' cent. — With this solution all but one specimen 
give the positive reaction. Specimens will follow the end of 
the pipette about the dish if it is moved slowly. This is done 
by a series of positive reactions. Specimens will give the 
complete food reaction on the end of the tube. 

Citric, 2 per cent. — Strong negative reactions. 

1 per cent. — Less marked negative reactions. Tendency 
to positive in some cases. 

Y*^ per cent. — Positive reactions in nearly all cases. Re- 
mainder indifferent. 

YX) per cent. — Indifferent. 

Citric acid in weak solutions seems to be a ver}^ ineffective 
sort of stimulus, not causing pronounced reactions of any kind. 

Formic, i per cent, and yV V^^ cent. — Prompt and de- 
cided negative reaction. Causes a resting worm to give 
a weak negative reaction of the anterior end, but does not 
start the whole animal into movement, provided the tube is 
withdrawn after the first reaction is obtained. 



652 EAYMOND PEARL. 

TTjy per cent. — Negative reaction, but decidedly less pro- 
nounced than with preceding concentrations. Does not 
cause any movement whatever in resting specimen. 

^ per cent. — Negative reaction, less strong than in pre- 
vious cases. In some cases positive reaction. Noticeable 
tendency to give slight positive reaction just before the 
definite negative response. 

•g\p per cent. — Well-marked positive response. 

Summary. — The same conclusions are to be drawn from 
the experiments on organic acids as from those on mineral 
acids, viz. that to strong concentrations of a given substance 
the negative reaction is given, while weak concentrations 
cause a positive response. Oxalic acid is rather peculiar in 
that it appears to furnish in all concentrations a stimulus of 
the proper quality to induce the positive response, but is at 
the same time excessively harmful in any above the weakest 
solutions. 

Al kalies. 

Sodium Hydrate, i per cent., j^jy per cent,,and ^V P^i" 
cent. — Immediate strong negative response. Specimens 
turn away very sharply. In — per cent, the reaction is 
slightly weaker than in the other two. 

J^ per cent. — Negative reaction. Stimulus applied to pos- 
terior end of body is sufficiently strong to cause crawling 
movement. 

yL per cent. — Weaker negative reaction. Sufficiently 
strong to start resting animal into movement. 

jjrjj- per cent. — Weak negative reaction. Ineffective on 
resting worm and on posterior end of body of moving 
specimen. 

-^^ per cent. — A^ery weak negative response. In one 
specimen sharp positive reaction ; performs whole food re- 
action on the end of the tube. 

-^^-^ per cent. — Positive reactions from all specimens. The 
complete food reaction is given. 



MOVEMENTS, KTC, OP FRESH-WATIOR PLANA RTANS. 653 

To solutions below this concentration the organisms are 
either indifferent or, in a few cases, weakly positive. 

Sodium Carbonate, -i- per cent. — Rather weak negative 
reaction, 

y\) per cent. — Majority of all reactions positive. Remainder 
weakly negative. 

Ya per cent. — Well-marked positive reaction in all cases. 
The specimens can be led around the dish by moving tlio tube 
slowly. 

Below this concentration the reactions were either in- 
different or weakly positive. 

Summary. — A caustic alkali (NaOH) and a salt of strong 
alkaline reaction (NaoCOg) produce essentially the same 
results as the acids. In strong solutions they cause negative 
reactions ; in weak, positive. 

Salts of Heavy Metals. 

Copper Sulphate, -^jj per cent. — At the very first 
trials the animals all turned sharply and immediately towards 
the stimulus (positive reaction), but the solution was so 
strong as to throw the animal into convulsions, when the 
head came very near the mouth of the tube. Subsequent 
trials produced the negative response, 

^ and -jjy per cent. — All specimens give positive reaction. 
The head is brought up to the tube, and the worm glides up 
over the latter. 

With all concentrations of CuSO^ there is a very well- 
marked local' contraction of that part of the bod}^ which is 
stimulated. 

In another series of experiments with this same substance, 
the strongest concentration which would produce the positive 
reaction in all cases was ^^ per cent. 

Zinc Sulphate, -g- and jjy per cent. — Sharp and imme- 
diate negative reaction, 

■jLy per cent. — Negative reaction, but less pronounced than 
in former cases. 

-^-^y and -Jy per cent. — Specimens stop when stimulated, 

VOL. 40, PART 4. NKW SElllES. U U 



654 RAYMOND PEAR];. 

and wave the anterior end about in the water, first 
away from and then towards the source of stimulation. 
As the head comes nearer to the end of the tube, where the 
solution is strongest, it is more strongly stimulated, and 
gives a definite negative reaction. As it gets out into the 
weaker zone again it is stimulated to a positive reaction 
once more. If the tube is now removed the specimen will, 
in some cases, after a short time turn sharply towards the 
place where it was, and move in that direction. In other 
cases the negative reaction finally predominates. It not in- 
frequently happens that in the earlier part of this reaction the 
anterior end only moves very slightly towards, or very 
slightly away from the stimulus, so that the body seems, at 
first sight, to be fixed in one position. The planarian, in this 
strenuous reaction, probably comes as near to the hypothe- 
cated behaviour of the famous "Buridan's ass" as anything 
is ever likely to in actual practice. 

y-iy- per cent. — One specimen gave clearly marked positive 
reaction in every case. Others as in the preceding solutions 
(tV P®^* cent, and -^-^j per cent.). 

■TT^jy per cent. — Well-marked positive reaction. Specimens 
give complete typical food reaction. 

In one case, with a small worm, I was able to produce 
craAvling in a backward direction by continuous stimulation 
of the anterior end in the middle line of the body with 
-yL per cent. ZnSO^. 

Summary. — The results from solutions of salts of two 
heavy metals are in accord with those obtained with other 
chemicals. 

Other Salts. 

Sodium Chloride, i per cent, and yV P^'^' cent. — Nega- 
tive reaction; distinct, but not as strongly marked as the 
negative reaction to strong acids. 

^\y per cent. — Weak negative reactions and weak positive 
reactions in about equal numbers. Many of the tiials produce 
no response whatever. 



MOVEMENTS, ETC., OF EKESII-WATRR PLANARIANS. 655 

-^^ per cent. — Weak positive reactions in nearly every 
case. No negative reactions. The typical, complete food 
reflex I have not been able to induce with sodium chh^ride. 

Concentrations below this do not produce any definite re- 
action. 

In general, NaCl is a very ineffective stimulus to pla- 
narians, either to the positive or the negative reaction. Dis- 
tilled water is a considerably stronger stimulus to the positive 
reaction. 

Sodium Bromide, 2 per cent. — Weak but distinct 
negative reaction in all cases. 

I" per cent. — Well-marked positive reaction in nil cases. 
Complete normal food reaction is produced. 

Potassium Chloride, 2 per cent. — The animals usually 
react in a pecnliar way to this and stronger solutions of KCl. 
When stimulated they stop, turn the anterior end either 
slightly towards or slightly awny from the source of stimula- 
tion, and then stay in the same place and squirm and twist 
the body. In some cases there is a well-marked negative 
reaction. 

i per cent. — Some specimens give negative reactions 
in the fii'st few trials ; afterwards give definite positive 
responses, ns do other specimens in all cases. In one case 
the specimen gave marked positive reaction, and after the 
head was turned towai'ds the stimulus, remained quiet in the 
same position as long as the chemical acted. 

y*y per cent. — All specimens give positive reaction or 
are indifferent. The whole food reaction took place on the 
end of the tube. In this experiment it could be clearly 
demonstrated that the })harynx is positively chemotactic to 
this substance. It is probably positively chemotactic to all 
substances which induce the preceding portions of the feed- 
ing reaction. If, after the pharynx had been extruded, the 
tube was turned about so that the ventral surface of the 
animal could be seen, and the posterior part of the body was 
moved with a needle, so as to change the position of the 
pharynx with reference to the mouth of the tube, it could be 



656 RAYMOND PEAEL. 

seen that tins organ bent directly towards the mouth of the 
capillary. The pharynx oriented itself with reference to the 
issuing chemical. 

The cases in which specimens were "indifferent" to this 
solution (i.e. did not give either the positive or negative re- 
action) were evidently not due to the fact that the animal 
was not stimulated^ hul, on the contrary, that it was stimu- 
lated about equally to negative and positive responses. This 
was indicated by their restless behaviour when " indifferent." 
While the animal as a Avhole moves in a straight line, the 
head constantly moves slightly towards and away from the 
stimulus. Evidently the solution is not quite strong enough 
to induce a definite negative reaction, nor quite weak enough 
to cause a clear positive response. 

■^jj- per cent., j\j- per cent., and -^j^ per cent. — Distinct posi- 
tive reaction in all cases. 

-jjrjj per cent. — Positive reactions in some cases, mainly 
indifferent. The " indifference " is now due to lack of 
stimulation. 

Below yiy- per cent. I have been unable to get definite 
responses of any sort with KOI. 

Magnesium Chloride, i per cent. — Usually sharp 
negative reaction. In some cases a slight turn towards the 
stimulus preceded the negative response, nnd in some few 
other trials the aniaial was indifferent. 

-i- per cent. — Weaker negative reaction. In one case 
clear positive reaction. No local contraction of the region 
stimulated is caused by this chemical. 

Jy per cent. — Positive reaction in all cases. Complete 
food reaction could be induced. 

_i_- per cent. — Weak positive reaction or indifferent. 

Summary. — To the salts NaCl, NaBr, KCl, and MgCU 
the planarians react as to other chemicals, by giving the 
negative response to strong concentrations and the positive 
to weak. 

Cane-sugar, — Sugar solutions, in all concentrations 
above j^ per cent., so far as I have been able to discover. 



MOVEMENT^;, ETC., OF l-'liliSlI-WATEU PLANAUlANt!. 057 

cause well-marked positive reactions in all cases. This is 
the ODly chemical which I have found that causes only one of 
the reactions. 

Distilled AV'ator. — To distilled water applied by the 
cnpillary method the organisms give a well-marked positive 
reaction in all cases. That the reactions to very dilute solu- 
tions of chemicals were not due to the distilled water in cases 
where this was used as the solvent, rather than to the chemical 
itself, was proven in the following way : — Parallel experiments 
were performed, using tap water as a solvent, and in every 
case the same reaction was given to the tap-water solution as 
to that in distilled water. At the same time the specimens 
would not react to clear tap water applied in the same way by 
the same tube. 

2. General Summary. — Putting all the results on the 
effects of localised chemical stimuli together, we are forced to 
the somewhat remarkable conclusion that practically all sub- 
stances are both '' attractive '^ and " repellent " to planarians. 
Evidently, then, the chemical composition of a substance is 
not of the first importance in determining how the individuals 
shall react to it ; but, on the contrary, its concentration is the 
important matter. To weak solutions of any chemical the 
animals give positive responses, while to strong solutions they 
give negative. 

Between the behaviour towards chemical stimuli and 
towards mechanical stimuli there is a very close parallelism, 
or, perhaps better, identity, which is evidently something of 
fundamental importance. In order to bring this out more 
clearly it may be well to arrange in tabular form the results 
of the study of the reactions to these two stimuli. 



658 



RAYMOND PEAKL. 





Mechanical Stimuli. 


Chemical Stimuli. 




Strong. 


Weak. 


Strong. 


Weak. 


Unilateral stimula- 
tion oF head region 


Negative re- 
action 


Positive 

reaction 


Negative re- 
action 


Positive 
reaction. 


Stimulation of lieiid 
region on median 


Either a very 
strong ne- 


Positive 
reaction 


Strong nega- 
tive reac- 


Positive 
reaction. 


line 


gative reac- 
t i n, or 
crawling 
backwards 




t i n, or 
crawling 
backwards 




Stimulation of middle 
region of body 


Essentially the same as 
for stimulation of the 
head 


The same as for stimula- 
tion of the head, except 
that the sensitivity is 
much less, and dimin- 
ishes more rapidly pos- 
teriorly than in case of 
mechanical stimuli. 


Stimulation of pos- 
terior region of 
body 


Crawling 
ahead 


Local con- 
traction 


Crawling 
ahead 


No effect, 
or slight 
local con- 
traction. 



From this close parallelism we must conclude, 1 tliiuk, that 
iu the behaviour of planariaus the ciualitative character of a 
stimulus is of little importance in comparison with its quanti- 
tative relations. Or, to express it differently, to all stimuli 
which are of low intensity the flat-worm gives the positive 
reaction, while to stimuli which are of high intensity it gives 
a negative response. This sort of behaviour will at once be 
seen to be, iu the long run, purposive, and is, further, of a 
kind which might very well have been developed by the 
action of natural selection. In the long run the planarian's 
reactions will take it away from injurious substances and 
into favourable surroundings. 

These results on chemicals are interesting in connection 
with the work so much done in recent times on the specific 



MOVEMIiNTS, ETC., OF FRESH-WATKIi PLANA inAN.S. 659 

effects of ions and the conclusions based on very fine (jii;inti- 
tative results with chemicals. Two such series of experi- 
ments as those quoted above from HCl and CuSO^ indicate 
what would be the worth of the assig'nment of an absolute 
value for the concentration of either of these two substances 
which would produce the positive reaction in planarians. 
Such instances might be multiplied, and they serve to bring 
out the fact, apparently so frequently lost sight of, that what 
an organism will do when stimulated is quite as much a 
function of the physiological condition of the organism itself 
at the time as it is of the stimulus. 

A comparison of these results with those of Yerkes (: 02) 
on the reactions of G on ion emus is of much interest. This 
author finds that though there is a well-marked and 
characteristic food reaction, which is given in response to 
food substances, whether in solid or liquid form, yet this 
reaction cannot be induced by other chemicals. It is stated 
that a number of chemicals were tried in all concentrations 
for the special purpose of detei-mining whether the food 
reaction might not depend upon intensity rather than quality 
of stimulus. This was not found to be the case. We must, 
then, conclude that Gonionemus is a stage farther along in 
its psychic development than is the flat-worm, for the medusa 
reacts with reference to the quality as well as to the in- 
tensity and location of the stimulus, while with the fhit-worm 
the intensity and location of the stimulus are by fai- the most 
important factors. It is necessary in the case of the flat- 
worm, to be sure, that there be mechanical and chemical 
stimuli acting together in order to produce the complex of 
reflexes forming the complete food reaction, thus indicating 
some relation to quality of stimulus. But for the production 
of what is, in one sense, the most important phase of the re- 
action, the turning towards the source of stimulation, the 
quality of the stimulus is not significant. 

With an understanding of the method of reaction to 
localised chemical stimuli, a number of interesting special 
problems present themselves. While it will not be possible 



6G0 RAYMOND PEARL. 

to take up all of tliem in this paper, a few of the specially 
important ones may be considered. 

One such important general question which arises is the 
problem of orientation to diffusing chemicals. Do planarians 
orient themselves along radial lines of diffusion and proceed 
towards the centre of diffusion ? It would seem that in the 
case of such a perfectly bilaterally symmetrical organism as 
PI an aria, if anywhere, Loeb's theory of orientation ought to 
hold good. This theory accounts for orientation by sup- 
posing that when an organism is stimulated unilaterally its 
motor organs are caused to act either more strongly or more 
weakly, as the case may be, on that side than on the other. 
This results in bringing the long axis of the body parallel 
with the lines of action of the stimulus; and then, since 
symmetrical points on either side of the body must be equally 
stimulated, the organism moves in a straight line towards or 
away from the stimulus. Jennings has shown (: 01) that for 
most stimuli this theory of orientation does not hold in the 
case of the Infusoria. 

From the account of the reactions of planarians to 
chemical stimuli which has been given, it will be at once 
seen that there is in this case, to some degree at least, 
an orienting reaction. With weak chemical stimuli the 
head turns towards the stimulus in such a way as to point 
the anterior end very directly towards the source of stimula- 
tion. It might be thought that this marked a pure orienta- 
tion, but it must be remembered that the organisms turn the 
head just as precisely towards the point from which a weak 
mechanical stimulus comes. The two reactions are evidently 
exactly the same thing. However, a single mechanical 
stimulus can hardly be considered a directive stimulus of 
the sort which induces an orientation, such as, for example, 
the electric current. The orientation of unicellular organisms 
to the constant current is the purest type of an orienting 
response, however, and the most characteristic thing about it 
is that the organism, after having the anterior end turned 
towards one of the poles, keeps the long axis of the 



MOVEMENTS, ETC., 01-' FKESlI-WATEll I'LANAKIANS. G61 

body pai'iiUol to the lines of action of the stimu- 
lus. This movement of the animal in a constant rehition to 
a constantly acting stimulus is^ as I understand it, the funda- 
mental criterion of an orientation according to the theory 
above mentioned. Now if we lind, as has been shown above 
to be the case, that the organism gives precisely the same 
reaction to a chemical unilaterally applied as it docs to a 
single weak mechanical stimulus similarly applied, it seems 
doubtful whether we can consider that there is such an 
orientation in the case of the chemical, even though the head 
is directed very precisely towards it. On the contrary, it 
seems apparent that we are dealing here with a well co- 
ordinated motor reflex only — such as, for example, the reflex 
of a frog's hind leg, which brings its foot very exactly to the 
point stimulated on the side of the body. 

A crucial test of this point may be obtained by submitting 
the animals to the action of some chemical to which they are 
known to give the positive reaction when it is applied locally, 
only arranging the experiment so that it is diffusing over a 
large area. Under these conditions, if the oi'ganism shows 
positive orientation, it ought to move along the lines of diffu- 
sion straight up to the source of diffusion. To test this matter 
I constructed a trough of the form shown in Fig. 33, I. 
On a plate of glass A was fastened the trough B, which was 
cut from a block of paraffin. The internal dimensions of this 
trough were 50 mm. x 50 mm. x 5 mm. Only the sides were 
of paraffin, the glass plate serving as the bottom. A hollow 
was cut in one end of the trough, and a glass tube D, about 
4 cm. long, was fastened into it in an upright position. Then 
from the point x on the inside of the trough a fine needle was 
thrust through the paraffin till it came out into the hollow 
previously cut in the wall. A sectional view of this part of 
the device is shown in Fig. 33, II. When it was desired to 
use the apparatus the trough was filled with filtered tap 
water and a number of planarians placed in it. Then into 
the tube D was introduced a certain amount of the solution 
whose effects were to be tested. By varying the amount of 



062 



RAYMOND PEAUIi. 



the solution introduced, the rate of its diffusion through x 
into the Wciter could be very nicely controlled. This matter 
was thoroughly tested, and the apparatus in a sense 
calibrated by the use of coloured solutions before the actual 
experiments were begun. 

A considerable number of experiments were tried with this 
diffusion trough, with the following results : — In no case was 
there any observable orientation of the organisms. A typical 
experiment will illustrate what actually took place. A 
^ per cent, solution of NaoCO., which by the capillary 
method always produces a sharp positive reaction, was put 




Fig. 1)3. — I. DifTusion trough used for testing tlie reactions of planarians 
to diffusing cliemicals. A, A. Glass base plate. B, B, B. Paraffin 
trough, x. Point of opening of diffusion tube. C. Cavity of 
trough in which the specimens are placed. D. Tube in whicii the 
solution to be tested is placed. II. Enlarged sectional view of the 
end of the trough bearing the diffusion tube. Lettering as in I. 

into the tube D in sufficient quantity to give a diffusion of 
moderate rate. After it had been diffusing for some time (by 
test with coloured solutions long enough to reach the middle 
of the trough) specimens were introduced at the end C. They 
started gliding about in random directions at once. Some 
passed diagonally up to the end D ; others remained nearer 
the end ; while still others went up on the paraffin sides to 
the end D. None 'went straight towards .p after they had 
come into the region where the chemical had diffused. No 
reaction of any sort was given in the course of the passage 



MOVEiM LINTS, KTC, OF Flib]SH-WATEli I'LANAKIANS. 663 

towards the end D in the majority of cases. lu some few 
instances an individual would give a weak positive reaction 
(i. e. turn slightly towards x) at some point in its course, but 
this was so small in amount that it did not in most cases 
turn the animal directly towards x. Further, the direction 
of movement was frequently changed considerably, and turned 
away from x after this weak positive response. In other 
words, the animals moved about in the trough practically at 
random, giving only slight reactions in a few cases while in 
the area of diffusion. Many of the individuals, after reaching 
end D of the trough, turned around and went back to the 
other end again, just as they would have done provided no 
chemical had been present. Other specimens would glide 
across the trough on the paraffin of the end D. Only these 
specimens showed any definite response to the chemical. 
AVhen they came within the length of their own bodies from 
the opening x they gave a well-marked positive reaction and 
went to X. Having arrived there, they explored and 
"gripped" the edge of the hole with the head, and then 
extruded the pharynx. The pharynx was usually stretched 
up into the diffusion opening, and the worm proceeded to 
feed for a time on NaoCOo. 

These experiments were repeated many times with a 
variety of chemicals of various concentrations, and diffusing at 
various rates. It was very certain in all cases that there was 
no definite orientation along lines of diffusing ions. When 
the organism by chance came near the diffusion opening x, it 
would give a positive reaction if the solution was of the proper 
concentration, and then proceed to give the complete food 
reaction over the hole, bat there was no continued orientation. 

There Avas a similar absence of a negative orienting 
response when strong solutions of acids were used. In this 
case the animals stayed at end C of the trough, but this was 
because when, in the course of their random movements, they 
struck the diffusing chemical where it was of sufficient con- 
centration, they gave the usual negative reaction, turning the 
anterior ends about 30"^ away, and starting off on the courses 



G6^ RAYMOND PEARL. 

SO defiued. It" they came in contact with the strong solution 
again the}' repeated the reaction. In no case did they turn 
squarely around with their heads directly away from x and 
the long axis parallel to the lines of diffusion. 

It would be unprofitable to further multiply accounts of 
these experiments, since all led to the same result. No 
definite orientation occurred, but only the positive and nega- 
tive motor reflexes coupled with random movements. 
Whether, as some maintain, we have in these positive and 
negative reactions the " Diuge an sich " of orientations is a 
question for the metaphysician rather than the physiologist 
to decide. The objective reality of the matter is that in the 
behaviour of planariaus towards chemicals there is no orien- 
tation in the lines of diffusing ions, i. e. no phenomenon like 
the orientation of Paramecium to the electric current. 

Another problem o£ importance in connection with the re- 
actions of the organisms to chemicals has to do with the 
formation of collections of individuals. Are collections 
formed in certain chemicals, as is the case with certain of the 
Infusoria as described by Jennings ? As this author has set 
forth, Paramecia will form dense aggregations in drops of 
various chemicals, particularly weak acids, introduced into 
the culture water. The method by which this is done is as 
follows: — Individuals swimming about at random strike the 
drop of acid by chance and pass into it without giving 
any reaction; when, however, they come to the opposite 
side of the drop, and start to pass from it to the water again, 
they are stimulated and give their characteristic motor re- 
action (jerk back and turn towards the aboral side). This 
reaction turns them back into the drop, which forms, as it 
were, a trap for all that enter it. In a short time a dense 
aggregation is formed. This is almost the only method of 
active reaction, known aside from orientation, which will 
produce collections of organisms in chemicals. Its essential 
feature is not the getting of the organisms into the chemical, 
this being purely a matter of chance, but the holding of 
them in the chemical after they have entered it, by what 



MOYKMENTS, ETC., OF FRESTI-WAl'KR rLANAlUAXR. 665 

amounts to a negative reaction to the suvronnding water. 
Tlie question, then, is, can we get any such formation of 
collections by the retention of those specimens wliicli have 
entered an area by cliance in the case of Planaria ? 

This problem was attacked in a nnmber of different ways, 
but the clearest results could be obtained by the " two-drop " 
method of Massart. Two drops of fluid of e(|ual siz(^ are 
placed near each other on a slide, and a narrow connecting 
band is made between the two by drawing some of the fluid 
across with a needle. One of them was usually of culture 
water, while the other was of the solution to be tested. Now 
evidently, if the animals form collections b}' the " motor 
reflex " method, they ought to pass into the drop of solution 
without any reaction, but when they attempt to pass back 
into the water drop they should be stimulated to a negative 
reaction and thus turned back. 

An experiment with a solution to which the animal gives a 
sharp positive reaction may first be reported. One of the 
drops was tap water, and the other was 1 per cent, sugar 
solution, to which the specimens gave a strong positive re- 
action. tSeveral small planarians were put into the water 
drop. They glided rapidly about this drop, and soon one 
came up to the bridge connecting the water with the sugar. 
It was headed sti*aight for the sugar drop, and passed over 
into it without any reaction whatever. Up to this point the 
behaviour is like that of the Infusoria towards the acid drop. 
This specimen circled about in the sugar drop, and after a 
time became directed towards the connection between the 
sugar and water, and passed back into the water drop with- 
out giving the faintest trace of a reaction of any sort. All 
the specimens passed back and forth betw^een the two drops 
without giving any reaction, except in some cases a weak 
positive one. The conditions under which a positive reaction 
is given are that a specimen should come more or less trans- 
versely across one end of the connecting bridge, as shown in 
Fig. 34. It then usually gives a weak positive reaction and 
turns slightly towards the other drop. It may do this on 



666 



EAYMOND PEARL. 



passing either from the water to the sugar or vice versa. 
When in sugar solution it gives a positive reaction to tap 
water, whether applied by the capillary tube method or as just 
described. It is evident, from this experiment, that collec- 
tions are not formed by planarians in the same way that they 
are by Infusoria. The animals are not negative to the 
surrounding water after they have been in the solution. To 
test and verify this conclusiou the experiment was repeated 
with solutions of diiferent substances. It was found that in 
case of all substances in concentrations to which the animals 
gave a positive response when stimulated by the capillary 
method, the specimens would pass back and forth from water 
to solution and vice versTi, indifferently. If solutions were 
used in concentrations to which a negative reaction was given 




Fig. 34. — Diat^ram showing the arrangement of " two-drop " experi- 
ment with chemicals. 



when stimulation was by the capillary method, the specimens 
merely stayed in the water drop. When they came to the 
boundary line of the strong solution they gave the negative 
reaction, and hence stayed in the water. This immediately 
raises the question, why would there not be a permanent 
collection of the planarians formed in a drop of^ a substance 
to which they give the positive reaction, provided they were 
First put in a drop of some substance to which they were 
strongly negative ? There is evidently no theoretical reason 
why this should not take place, but there is an important 
practical one. This is that any solution which would cause a 
negative reaction, under these circumstances, will, so far as I 
have found, also seriously modify the animals' movements, if 



MOVEMENTS, lOTO., OF ERE,S1I-WAT1<:R PLANA RIANS. 667 

they are immersed in it. They will simply squirm about autl 
make no progressive movements, and hence not get into 
the drop of substance to wliich they are positive. But it is 
quite possible that by making a long enough scries of experi- 
ments on this point, one might get a solution just strong 
enough to cause a negative reaction, and in which the 
organisms would still move well. We would then get a 
collection in the positive drop. The important thing, how- 
ever, is that to the water in which they live tlie animals do 
not, under any circumstances, give a negative reaction, and 
hence under normal conditions no collections can be found by 
a "motor reflex" method. 

It may be well, before leaving this subject, to point out the 
fundamental physiological difference betAveen the Infusoria 
and the planariaus, on which the difference in the behaviour 
towards chemicals is based. It is that in the case of the 
Infusoria there is but one form of reaction (the "motor reflex" 
turn towards a structurally defined side) regardless of whether 
the stimulus is strong or weak, while in the case of the 
planarian there is a qualitatively different reaction to strong 
stimuli from that which is given to weak. When the in- 
fusorian passes into the drop of acid it is apparently not 
stimulated at all (for what reason we do not know). When 
it attempts to pass from acid to water it is given a stimulus 
which must be in the nature of things a rather weak one, yet 
it responds with the only reaction it has, and is, as a 
consequence, kept in the acid. With the planarian any slight 
change in environmental conditions gives a weak stimulus, 
and the specimen turns towards the source of stimulation. 
This serves, together with random movements, to get it into 
the drop of solution ; but when it strikes again the water, 
which again must furnish a weak stimulus, it gives the same 
positive reaction and passes out into the water. The ability 
to differentiate in the reactions between the strong and weak 
stimuli gives the organism a far greater range in its 
activities. 

Another problem which is of ititerest in connection with 



668 RAYMOND PEARL. 

food and clieiiiical reactions is the relation of the condition 
of tlie organism as regards liuiiger to its reactions to stimuli. 
It might be supposed that an individual which had not had food 
for some time would be more apt to give the positive reaction 
to a given stimulus than one which had just fed. 

To test this point parallel experiments were instituted with 
specimeus allowed to feed till they left the food spontaneously 
about three hours before the experiments, and specimens 
which had been kept for three weeks in a dish of clear water. 
NaBi- was used as the stimulating solution, and Avas applied 
by the capillary method. The specimens chosen were of the 
same species, P. dorotocephala, and as nearly as possible 
of the same size. The only difference which could be detected 
between the fed and the unfed animals in their behaviour 
towards a f per cent, solution of NaBr was that the unfed 
animals gave the whole food reaction on the end of the 
capillary tube, while the recently fed specimens only went so 
far as to give the positive reaction, and touched the end of 
the tube with the anterior end of tlie head. They did not 
"grip" it and pass up on to it, as did the others. In the 
main point at which I was working, namely, the giving of 
the definite positive reaction, there was no discoverable 
difference between the fed and unfed specimens. One set 
gave the reaction just as promptly and decidedly as did the 
other. Next a weaker solution, y\y per cent., was tried. 
With this solution about 50 per cent, of the specimens in 
ordinary condition give a v/eak positive reaction, and 50 per 
cent, are indifferent. This concentration, being about on the 
border line betAveen that which affords no stimulus at all and 
that Avhicli is a definite stimulus for the positive reaction, 
ought to bring out any differences which may exist between 
fed and unfed individuals in the sensitivity to stimuli for the 
positive reaction. As a matter of fact, no difference in the 
behaviour of the two sets was to be observed. One gave a 
well-marked positive reaction in as many cases as did the 
other. In some instances the reaction time of the fed 
specimens seemed to be slightly greater than that of the 



MOVEMKNTS, KTC, OK FRESH-WATKR PLANARIANS. 669 

unfed, but this was neither marked nor of general occur- 
rence. This experiment was afterwards repeated with other 
specimens, and with sugar as the stimuhiSj with essentially 
the same results, I have also repeatedly tried stimulating 
with various solutions specimens which had just ceased 
feeding, and in these cases found no certain difference 
between their behaviour and that of specimens whicli had 
not been fed for some time, with regard to the giving of the 
positive reaction. It would appear, then, that so far as tlie 
giving of the positive response to weak stimuli is concerned, the 
amount of food the animal has previously had is of very little 
consequence. The failure of fully fed specimens to give the 
full feeding reaction on the end of the capillary tube indicates 
that the physiological changes induced by recent feeding 
affect the performance of the food-taking rather than the 
food-seeking reflexes. 

3. Unlocalised Action of Chemicals. — An extensive 




Fig. 35. — Diagram showing tlicfonu of crawling niovcnient, exhibited 
by PI a 11 aria wlieii placed in 10 per cent. NaCl. 

series of experiments on the effects of immersing planarians 
in various solutions was performed, but as the results threw 
but comparatively little light on the general nature of the 
behavioui', they will be reported only briefly. Immersion in 
any strong solution causes marked changes in the move- 
ments. The gliding is made very much slower or entirely 
disappears. In 10 per cent. NaCl a peculiar form of crawling 
appears. Very pronounced contraction waves pass over the 
body longitudinally, giving it the aj)pearance shown in 
Fig. 35. In 2 per cent. CuSO.j^ the animals make no pro- 
gressive movements, but wave the head violently from side to 
side. In strong solutions of acids the worms squirm violently 
without making any effective progressive movements. In all 
these strong solutions the sensitiveness to all stimuli is 

VOL. 46, PART 4. NEW SERIES. XX 



670 RAYMOND PEARL. 

greatly diminished. This can best be shown with mechanical 
stimulation. In strong solutions of NaCl (10 per cent.) the 
animals make no attempt to right themselves if placed with 
their dorsal surfaces down. Another peculiar effect of strong 
solutions of NaCl is to cause the extrusion of the pharynx. 
This organ is thrust out of the body and extended to a much 
greater length than is usual. Immersion of the animal in 
weak solutions that cause the positive reaction — as^ for 
example^ 1 per cent, sugar — has no definite effect on the 
movements, but when in these solutions the animals will give 
the positive reaction to tap water when the latter is applied 
by the capillary tube method. Under such circumstances 
contact with water is a slight environmental change, and acts 
as a weak stimulus. 

III. Tliigmotaxis and the Righting Reaction. 

a. Tliigmotaxis. — If a specimen of Planaria is turned 
over and placed dorsal side down on the bottom, it will 
immediately right itself. This is done by a very characteristic 
reaction, and is one of the first things to attract the attention 
of one studying the behaviour of the organism. Loeb ('94, pp. 
251 — 252) held that the righting reaction in the poly clad 
Thysauozoon was due to the negative and positive tliig- 
motaxis {" stereotropism ") of the dorsal and ventral surfaces 
respectively. The evidence offered for this view was that 
when the thigmotactic relations of these two surfaces were 
reversed, the animal reacted strongly, and tliat this result 
could not be due to any effect of gravitation, since the animal 
assumed all possible relations to gravity, and kept them for 
considerable periods of time. It seemed to me desirable to 
get, if possible, some further evidence on this subject, and to 
work out the mechanism of the righting reaction. 

That the dorsal surface of the animal is negatively thigmo- 
tactic is certain, and can be shown in other ways than by 
laying the animal on its dorsal surface. For example, if a 
piece of cover-glass be gently laid on the dorsal surface of 



MOVLIWENTS, ETO., OF FRESH-WATER PLANAKIANS. 671 

either a resting or a moving' specimen, it will very promptly 
move out from under it. Furtlier, if crevices are arranged of 
this form (^^r"^) by supporting cover-glasses at two corners, 
and letting the two opposite corners rest on the bottom of the 
dish, specimens will not go into them. The moment the 
dorsal surface touches the cover-glass above, the worm 
begins to react violently, changing its direction of movement, 
and goes out from under the cover. 

With the existence of an apparent negative thigmotaxis of 
the dorsal surface established, however, there still arises the 
question as to whether this is the sole cause which induces the 
inverted animal to right itself. The following experiment 
throws light on this point : — A specimen is placed ventral 
side up on a dry spatula in the air, and then the spatula is 
placed just beneath the surface of the Avater in a tall jar or 
large test-tube and quickly pulled out from under the worm, 
so that the latter starts falling through the water in an in- 
verted position. Another Avay in which the worm may be 
started falling ventral side up is by holding it on a scalpel 
point above the water, and then dropping it beneath the 
surface in the desired position. Before the worm has 
dropped any great distance it will give the characteristic 
righting reaction, and turn itself over so as to bring the 
ventral side down again. This is done in precisely the same 
way as when the animal is inverted on the bottom (to be 
described later). After the falling animal has thus righted 
itself it may again give the same reaction, and thus turn 
itself over so that the dorsal side is down again. In a few 
cases I have seen a worm after righting itself the first time 
keep right side up during the remainder of the fall. The 
most usual behaviour is for the animal to keep giving the 
righting reaction all the time that it is falling, although this 
does not, of course, keep it all the time with the same side 
uppermost. I have performed a large number of these 
dropping experiments in which the animals were started in 
both upright and inverted positions, and in all cases they 
gave the righting reacting one or more (usually more) times 



672 RAYMOND TEARL. 

before reacliing the bottom, provided the distauce through 
which the drop was made was greater than 7 — 10 cm. This 
result seems to indicate that there is something more con- 
cerned in the righting reaction than the negative thigmo- 
taxis of the dorsal surface for the following reasons : — (1) the 
dorsal surface is not in contact with any solid of this experi- 
ment ; (2) it is in contact with water onl}', just as is normally 
the case when the animal is right side up. It may be 
objected that the experiment is not conclusive, because, as a 
result of the falling, there is an increased water-pressure on 
the dorsal surface, and this may act as a thigmotactic 
stimulus. This objection is met by two different facts. 
First, the animal gives the righting response in some cases 
while falling ventral side down, under which circumstances 
there can be no increased pressure on the dorsal surface. 
Second, if a stream of water from a pipette is directly 
squarely against the dorsal surface of a worm normally 
gliding about on the bottom the righting reaction is not 
induced, regardless of the force of the stream. Evidently 
this stream of water against the dorsal surface produces a 
pressure on the dorsal surface similar to that when the 
animal is falling, and if the righting reaction in the falling 
is due to increase of pressure on the dorsal surface, we might 
suppose that some indication of it would be produced in this 
case. As a matter of fact it is not. We must conclude, 
then, that the righting reaction is due, at least in very large 
part, to some other cause than the negative thigmotaxis of 
the dorsal surface. This is indicated also by the fact that 
when solid bodies are laid on the back of a specimen in its 
normal position, the reaction which is caused is not the 
righting action, as would be expected if the latter were 
due solely to the negative thigmotaxis of the dorsal surface. 
The righting reaction is clearly not due to gravitation, since 
the flat-worms move on the surface film with the dorsal 
surface downward. This leaves, as the only factor to which 
the reaction can be due, the positive thigmotaxis of the 
ventral surface. I am convinced that it is to this factor that 



MOVEMENTS, ETC., OF FRKSH-WATER PLANARFANS. G73 

the reaction is chiefly due. While the negative thigniotaxis 
of the dorsal surface plays some part in the reaction, it is, as 
the experiments described above show^ a comparatively un- 
important factor. The specific relation of these two factors 
to the definite righting reaction will be brought out in the 
next section, in Avliich the form and mechanism of this 
reaction will be set forth. 

h. The Righting lieaction. — The righting reaction is 
a very characteristic piece of behavioui', and can best be 
described in a single phrase by saying that when the animal 
is placed on its back it throws itself into a spiral in such a 
way that tlie ventral surface of the head comes into contact 
with the bottom. This ventral surface then attaches itself to 
the bottom by means of the mucous secretion, and starts 
gliding ahead. As it goes forward it unwinds the remainder 
of the spiral, as each successive posterior part of the ventral 




Fig. 36. — Showinpr tlie form taken by Planaria in the righting reaction, 

surface comes into full contact with the bottom. The form 
of this spiral just after the ventral surface of the head has 
come into contact with the bottom is shown in Fig. 36. The 
spiral is thrown very quickly after the dorsal surface touches 
the bottom, and usually includes the whole length of the 
body at once. However, by observing a specimen in which 
it takes place a little more slowly than usual, it can be seen 
that the movement is started at the anterior end, 13eginning 
with, for example, the right side of the head, this is turned 
under, while at the same time the left side is raised. This, 
of course, brings the ventral surface of the head region down, 
and at the same time makes a twist in the body, just back of 
the head. In some cases this is the only twist that is made, 
while in others another similar twist is thrown in the body 
farther back. As the anterior end after it is righted glides 



674 RAYMOND PEARL. 

ahead, the spiral is nnwouiul by the raised edge of each 
twist dropping down and attaching to the bottom as soon as 
it is in a position where this is possible. Thus, of course, 
when the animal has traversed a distance equal to its own 
length it will have come entirely into the normal position 
again. The reaction is really a rotation of the body on its 
long axis through 180°. The mechanism of the turning is 
such that only a part of the body rotates at a time, — first 
the anterior end, then the portion next behind that, and so 
on, till the whole animal has turned over. This rotation by 
sections, as it were, causes the spiral form which the animal 
takes on in the reaction. 

The number of turns into which the body is thrown in 
forming the spiral varies Avith the length of the individual, 
and appai'ently to some extent witli its physiological con- 
dition. There may be only a half-turn in the whole body, or 
there may be one complete turn ; or, again, one and a half 
turns; or, finally, as many as two complete turns in the body. 
One complete or nearly complete turn, as shown in Fig. 36, 
is the usual form of the reaction. In large individuals 
more twisting is frequently seen. Evidently all the twisting 
that is absolutely essential for the righting of the specimen 
is the half-turn given by the turning of the anterior end 
ventral side down. 

The determination of the direction in which the spiral is 
thrown, or, in other words, the side of the body towards 
which the anterior end turns in order to get right side up, 
was for some time a very puzzling problem. A collection of 
statistics on the matter showed that the anterior end twisted 
towards the right and towards the left ^ in an approximately 
number of cases. This is precisely the result which would 
be expected if the matter were due to chance only, but the 
reaction did not give the appearance of being a chance 
matter. Finally, the determining factor was found to be the 
relation of the dorsal surface to the bottom. A cross-section 

' Tn tlie figure (Fig. oH) tlie worm is repiTseiifed willi (lie spiral tlu-owii 
towards tlie left.. 



MOVEMENTS, RTO., OF FRESM-WATRIl PLANARTANS. G75 

of tlie body of Plan aria has tlie form sliowii in Fig. 37. It 
is convex in outline on the dorsal side, and nearly straight on 
tlie ventral. As a consequence of this shape of the dorsal 
surface the animal when placed in an inverted position very 
seldom lies exactly on the mid-dorsal line, and if it does at 
first it almost immediately tips over to one side or the other, 
so that its cross-section has the relation to the bottom shown 
in Fig. 37, B and C. It h then found that the side of the 
body which is in contact with the bottom determines in 
which direction the spiral shall be thrown. If the right 
side of the dorsal surface is down the right side of the head 
will turn nnder towards the left and the left side will be 
raised up over towards the right, or, in other words, the 
head as a whole will rotate from I'ight to left, i. e. in a 

Dorsal 



Vfntral 
A 



B C 

Fig. 37-— Diagranimalic cross-section of Plaiiaria t,o sliow the contact 
relations of the dorsal surface of the body to Uie substrate in llie 
case of a specimen in an inverted position. 

counter-clockwise direction. If the left side of the dorsal 
surface of the body is down at the beginning, the head will 
rotate from left to right. This relation may bo made out 
easily by direct observation in all cases where the reaction is 
not too rapid. 

The righting reaction is a fnirly rapid one. The head is 
turned over and the spiral thrown in the case of a normal in- 
dividual almost immediately when the dorsal surface touches 
the solid. The length of time which it takes a specimen to 
get completely righted evidently depends on the length of 
the body, because the longer spiral which must be unwound, 
the more the time which must be taken. The following 
figures will bring out this relation between the size of the in- 
dividual and the time taken in righting. In ten trials with 



676 RAYMOND PEARL. 

an active but large specimen (about 12 mm. long) of 
P. dorotocepliala the average time taken to regain com- 
pletely the normal position after being inverted was 8'68 
seconds. With a small specimen (5'5mra. long) the average 
time taken in righting in ten trials was 5*22 seconds. The 
time taken in the reaction also depends, of course, on 
the general physiological condition of the animal. Thus in 
ten trials with a sluggish specimen, approximately 9 mm. 
long (thus shorter than the first specimen mentioned), the 
average time taken in regaining- the normal position was 
10'90 seconds. 

The thigmotactic irritability may be modified or reduced 
in several ways, and, as a consequence, the righting reaction 
will disappear entirely or in part. One of these cases has 
been mentioned above (p. 670) where it was shown that a 
specimen placed on its back in a 10 per cent, solution of 
NaCl makes no attempt to right itself. Similarly a specimen 
put in an inverted position on a dry surface, care being taken 
that no water surrounds the animal, will not give the righting 
reaction. In both of these cases the specimens are able to 
move. 

The Mechanism of the E-eaction. — It is a very 
difiicult matter to determine exactly the muscular mechanism 
of this righting reaction, since it is such a complicated move- 
ment, and is ordinarily done in its most essential feature — 
the formation of the spiral — so very quickly. Furthermore, 
as will appear from the operation experiments to be described, 
it is almost impossible to devise crucial experiments of a 
character which will demonstrate what the mechanism is. 
What I shall do, then, will be to present a tentative explana- 
tion of the mechanism of the reaction, together with the 
evidence for it which I have been able to obtain. I may say 
that the view to be presented is the result of a long and 
careful study of the phenomena both in normal and operated 
worms, and I believe that it is a correct explanation. 

The mechanism of the righting reaction is probably as 
follows : — The half of the body of an inverted specimen which 



MOVEMENTS, KTC, OF FRESH-WATEE PLANA RIANS. 677 

is in contact with the bottom extends (by the mechanism 
previously described, pp. 556, 557) in response to the stimulus 
given by the contact of the dorsal surface of that side of the 
body with the bottom. At the same time the opposite half of 
the body, by active muscular contraction, keeps its length 
the same. Thus any bending of the body away from the 
side stimulated as in the ordinai-y negative reaction is pre- 
vented, or, in other words, the long axis is kept straight by 
the opposite side maintaining actively its normal length. 
Now the necessary mechanical result of keeping one side of 
a flexible system at a constant length while the other side 
lengthens must be that the lengthening side will be thrown 
into a series of waves. In other words, it is mechanically 
impossible for the lengthening side to keep its whole edge in 
the same plane. Furthermore, if in such a system it is 
possible for rotation about a longitudinal axis to occur, the 
system will be thrown into a spiral of the form which the 
planarian takes in the righting reaction. Again, as soon as 
one side of such a system under elongating stress changes its 
level with reference to the remainder of the system, and thus 
starts the formation of the spiral, the long axis of the 
system (i. e. the centre of the spiral) will keep itself straight. 
Any further force elongating one side will merely throw the 
spiral into tighter coils without having any tendency to bend 
its long axis. This fact is of importance in the case of the 
planarian where the maintenance of the initial straightness 
of the long axis is done by the opposite side of the body. Of 
course, a symmetrical spiral cannot be formed unless the two 
edges are of equal length, but the moment the spiral of the 
planarian is started all necessity for one side keeping a con- 
stant length ceases. It must be kept in mind, however, as 
has been indicated above, that the force which produces the 
spiral must act on one side only, and hence the side of the 
planarian opposite that initiating the movement must be 
moved passively by the other in the spiral formation after 
this has once begun. The direction in which the spiral shall 
turn will evidently not be determined by the mere lengthen- 



678 RAYMOND PEAT?L. 

ing of one side of the body. The determinant of this is 
evidently a difference of tension on the uppei^ and lower sides, 
the spiral turning towards the side of greatest tension,^ This 
greatest tension is evidently, then, in the normal reaction on 
the dorsal surface, as we should expect on a priori grounds, 
since that is the part directly stimulated. 

To sum up, the spiral righting reaction of the planarian, 
as I have worked it out, is due to an elongation of that side 
of the body whose dorsal surface is in contact with the solid, 
while the opposite side of the body actively maintains its 
original length. As the elongation occurs the various parts 
of the body rotate freely about its long axis, and hence the 
whole worm takes on the spiral form. The spiral turns 
towards the dorsal surface in every case (i. e. so as to bring 
the ventral surface of the head down), as a result of the 
greater tension of the dorsal musculature on the elongating- 
side. 

The reaction is thus seen to be of almost the same cha- 
racter as the ordinary negative reaction to strong mechanical 
stimuli, in that the primary reaction is an extension of the 
side stimulated. The difference between the two is that in 
one case there is a bending of the longitudinal axis of the 
body, while in the other there is a rotation about this axis. 
On the view just given of the mechanism of the righting 
reaction the specific parts played by the positive and negative 
thigmotaxis of the ventral and dorsal surfaces are evident. 
The positive thigmotaxis of the ventral surface is the primary 
cause of the whole reaction, and is evidently the stronger 
factor of the two, as shown by the experiments of laying 
solid bodies on the dorsal surface of the animal when in a 
normal position. It will be recalled that such treatment does 
not call forth the specific righting reaction. Further 
evidence of this same thing is found in the fact that speci- 

' The statements as to tlie mechanical principles of a spiral have been veri- 
fied with different sorts of models, including plastic clay, rubber bands, etc. 
Lack of space will not permit the enumeration of these experiments in detail, but 
anyone can verily for iiimself the various statements with very little trouble. 



MOVEMENTS, ETC., OF FRRSII-WATER TLANARTANS. G79 

mens will remain in the normal position on tlio bottom of a 
clisli when there is a layer of plant debris a half-centimetre 
in thickness above them, and necessarily in contact with the 
dorsal surface. The negative thigmotaxis of the dorsal 
surface plays its part in the righting reaction in determining 
in which direction the turning shall take place. 

It has so far been shown that the view of the mechanism 
of the righting reaction presented is in accord with all the 
mechanical principles necessary to produce the observed 
results. The attention may now be turned to an examination 
of the evidence that this mechanism is the one which actually 
brings about the reaction. This evidence is obtained from 
experiments with worms on which operations have been per- 
formed. Obviously, if the mechanism described is the one 
by which the reaction is produced, any operation which 
destroys or throws out of working order any essential part 
of the mechanism will cause the typical reaction to disappear, 
or be greatly modified. 

We may first consider the reactions of the pieces resulting 
from cutting the animal in two transversely in the middle of 
the body. It is found that each of the pieces resulting from 
such a cut will perform the righting reaction in the typical 
manner. The spiral is formed, but there is usually only one 
half-turn of the body, i. e. just enough to bring the anterior 
end ventral side down. This then attaches itself to the 
bottom and starts gliding*, unwinding the spiral just as under 
normal circumstances. There is observable the same rela- 
tion between the side of the body, which is in contact with 
the bottom and the direction of the turn as in the normal in- 
dividual. The only striking diiference in the behaviour of 
the anterior and posterior pieces is that the reaction time of 
the former is much shorter than that of the latter. The 
anterior piece rights itself practically as quickly as does the 
normal animal, while the posterior piece took in one series of 
experiments 1 minute and 38*1 seconds (average of ten trials) 
for complete righting. This slower righting reaction is 
another expression of the generally lowered tonus of such 



680 RAYMOND PEARL. 

posterior pieces. By varying tlio position of tlie cuts, seg- 
ments of the body of various lengtlis may be obtained. All 
of these, which are about 1-|- mm. in length, will usually right 
themselves by as close an approximation to the typical spiral 
reaction as is possible under the circumstances. The side of 
the body which is lowest can be seen to elongate in these 
very short pieces, and just enough of a twist is found to 
bring the ventral surface of one corner of the anterior end 
into contact with the bottom. Of course, no complete spiral 
can be found in such short pieces. Their reaction time is 
very slow. 

Next, experiments were tried with the pieces resulting 
from splitting longitudinally anterior halves of worms in the 
middle line. These pieces had the form shown in Fig. 38. 
Evidently such pieces have only a half of the mechanism 
necessary for the performance of the spiral righting reaction, 



Fig. 38. — Operation diagram (see text). 

according to the view given above, and therefore should not 
be able to give the typical response. They have one com- 
plete side which may elongate, but they have no other side 
to keep the middle line straight, and so make the elongation 
eifective in forming a spiral. Such pieces, when placed with the 
dorsal surface down, reacted immediately by bending strongly 
towards the cut side, i. e., so that the concavity was on the 
cut side. This was kept up for a time, the animal squirming 
about violently, but it was finally replaced by another reaction. 
The ventral longitudinal muscles contracted strongly, and 
raised the anterior end of the piece well up from the bottom 
(shown in side view in Fig. 39, a) . After a strong raising con- 
traction the piece would extend and settle back again. Then 
after a time the raising was repeated, and it soon became 
noticeable that the piece was rising higher each time and 



MOVEMimTS, ETC., OF FRMSH-WATER PI,ANARIANS. 681 

settling back loss after each trial. Successive stages of this 
rising are sliown in Fig. 89, h, c, d. Finally, it worked up till 
it stood directly on the posterior end [e], and then the next 
contraction caused it to fall over of its own weight and come 
down right side up (/). The sticky mucous secretion at the 
posterior end was undoubtedly what held the piece up after 
each successive trial. This behaviour, as described, was 
uniform in all the trials. 

The behaviour of these pieces brings out several points of 
importance. First, it is to be noticed that no trace of the 
typical spiral righting reaction is to be seen ; yet, on the other 
hand, we find the pieces bending strongly towards the cut 
side when first inverted, which is just the ell'ect Avhich would 





a^ 



.cdUL 



f 

Fig. 3'J. — Diagram sliowiiif; the nietliod of riirliliiig adopted l)y one 
of the pieces shown in Eig. 38. 

be produced by the lengthening of the stimulated side in the 
normal righting reaction, provided, as actually obtains in this 
case, there was no opposite side to keep the long axis of 
the piece straight. Tlius we get precisely the result 
which would be expected if the view given of the mechanism 
of the reaction is the correct one. Another fact that is 
brought out by this experiment is the apparent adaptation 
shown. When the animal is unable to give the usual reaction 
for righting itseK it very quickly reacts in an entirely 
different way, but attains the same end result. 

A worm was cut so as to give a piece of the form shown at 
A in Fig. 40. This piece was placed in an inverted position 



682 



EAYMOND PEAEI;. 



and its reactions observed. Evidently, so far as injury of 
the mechanism by the operation is concerned, such a piece 
is in essentially the same condition as the pieces described 
in the previous expeimnent. It has only one complete side 
of the body. The piece when inverted scpiirmed about consider- 
ably at first, but gave no indication whatever of the normal 
spiral reaction. In a short time the violent movements ceased, 
and a notch was noticed in about the middle of the uncut 
edge (cf. Fig. 40, h). This soon grew larger, and extended 
more and more towards the ends of the piece, as shown in 




l'"iG. iO. — a. Operation diagram, ileavy lines indicate tiic cuts. 0, c, 
and d. Successive stages in tlie righting reaction of the piece A of 
diagram a. e and /. Cross-sections through A at two successive 
stages in the rigliting process. See text for further explanation. 

c and d. By close observation the cause of this appearance 
was found to be that the thin mobile edge was folding 
under and attaching its ventral surface along the bottom. A 
cross-section through the worm at this stage had the outline 
shown in e. As soon as a considerable portion of the edge 
had so folded under and become attached, the piece gave 
a series of strong contractions and literally " flopped " over 
the attached edge and came down right side up. A stage in 
this process is shown in cross-section in /. This behaviour 
was so peculiar, and at the same time precise, that the 



MOVEMENTS, ETC., OF FUBSH-WATEll I'LANAUIANS. 683 

experiment was repeated many times on this piece and on 
others cut in the same way. Tlie same method of" righting 
was always observed. After the first few times the turn 
is made in this way; it is done more (piickly at each succes- 
sive trial. 

This experiment leads to the same conclusion regarding 
the mechanism of the righting reaction as did the previous 
one. It affords another and more striking example of regu- 
lation in reactions. The piece attains the end (normal 
position) by a reaction which it undoubtedly never had 
occasion to practise before. 

Isolated longitudinal halves of the body react in the same 
way as did the piece described in the |)receding e.\})erinient. 
They right themselves by folding under the edge, and then, 
by violent contraction, drawing the rest of the body up over 
it. There is no trace of the spiral righting reaction. 

A specimen cut in the manner shown in Fig. 41 shows a 




Tig. 41. — Operation diagram (see text). 

very peculiar righting reaction. When placed dorsal side 
down the portion posterior to the median longitudinal 
slit immediately gives the spiral righting reaction, and drags 
the two ])assive anterior pieces over. The process is slow but 
very characteristic, so that there is no doubt of the nature of 
the reaction. This sIioavs that in that part of a single piece 
of a worm where the necessary mechanism is present we get 
the spiral righting reaction, while in other parts it does not 
appear. 

The same point can be brought out by splitting a worm 
longitudinally from the posterior end up to a point near the 
head. The complete anterior part of such specimens gives 
the normal spiral reaction, while the posterior parts remain 
passive so far as this reaction is concerned. 

A considerable number of different experiments were per- 



684 EAYMOND PEAEL. 

formed for tlic purpose of testing the righting reactions after 
operations, but since none of them bring out anything 
different in principle from the results already given, they will 
not be reported here. But it may be said in general, that all 
the experiments gave the same results with reference to the 
mechanism of the reaction, namely, that so long as the 
mechanism described above was intact the typical spiral re- 
action was given ; when this mechanism was destroyed or 
injured the reaction was not given, but the animal, if it 
righted itself at all, did it by a different method. 

When the animal falls freely in the water the righting re- 
action is induced because the ventral surface is no longer in 
contact with a solid. There is no reason for thinking that 
the mechanism of the reaction in this case is any different 
from what it is when the animal is placed in an inverted 
position on the bottom. The direction in which the spiral is 
thrown in the case of the falling animal is probably deter- 
mined by slight differences of pressure on the two sides of the 
body. 

c. Summary. — The flat-worm is positively thigmotactic on 
its ventral surface, and negatively thigmotactic on its dorsal 
surface. As a result of this it gives a characteristic righting 
reaction whenever the normal relations of either surface are 
changed. This righting reaction consists in throwing the 
body into a spiral in such a way as to bring the ventral sur- 
face of the anterior end down into contact with a solid (in all 
cases except when the animal is dropped into free water). 
The anterior end starts gliding and unwinds the spiral, 
thus righting the whole body. The thigmotactic reaction 
may be modified by chemical and other stimuli. All the 
evidence shows that the spiral righting reaction is due to a 
lengthening of the side whose dorsal surface is in contact 
with a solid, while the other side of the body keeps the long 
axis straight. The direction of the turn in the spiral is 
determined by the side of the body which is in contact with 
the solid. This reaction is thus seen to be closely related to 
the negative reaction to mechanical and chemical stimuli, so 



MOVKMKNTS, ETC., OF PRKSII-WATKIJ I'l.AXAElANS. G85 

far ;is iiun'linnisni is ooiict'Tiiod. Cut ])ioc{'s, in wliicli tlic 

nurinal ineclianisin for tlic rio'litiny reaction lias been 

destroyed, rig-ht themselves in various ways, thus showing a 
sort of regulation in reactions. 

lA'^. El ec trot axis. 

In view of the sliarp and precise reactions of ]^l;inarians to 
other stinnili, it was thought tliat they would furnish excellent 
oljjects for tlie study of electnjtaxis, Init nnfortnnntely this is 
not the case. Their reactions to the constant current are nf)t 
clear-cnt, since the specimens become wholly or partially 
paralysed in a very short time after the current begins to 
act, and as a consequence the reactions become feeble and in- 
distinct. For tho sake of completeness, however, and since 
some facts of importance are brought out, the experiments on 
this subject will be briefly reported. 

a. Methods. — The following methods were used: — The 
constant current used was obtained from the general lighting 
circuit of the University, and reduced to the proper intensity 
by interposed resistance. This apjiaratus for getting the 
current I have described fully elsewhere (: 00, : 01), so that 
it need not detain us here. In the circuit a rheostat was 
inserted for regulating tlie strength of the current. Ordinary 
unpolarisable brush electrodes were used. The specimens 
were placed either in a trough with clay ends, to serve as 
poles, and with paraffin sides of 5 mm. depth, or else on a 
slide under a cover supported by several layers of moistened 
filter-paper. These filter-paper ends then serve as the poles 
of the preparation, the brushes of the electrodes being laid 
upon them. The layer of water in which the specimens were 
in this sort of a preparation was approximately 2'5 mm. in 
thickness. Identical results were obtained by both the trough 
and the filter-paper method, but since tho latter is the lu^ater 
and generally more satisfactory method, it was used almost 
entirely in preference to the trough. 

h. Results. — The typical result of the action of the 
current on specimens in such a position that the long axis of 

VOL. 46, PAKT 4. NEW SERIES. Y Y 



686 



RAYMOND PEARL. 



the body is approximately at riglit angles to the direction of 
the cnrrent, niny be described first. If a number of speci- 
mens are g-liding- about at the normal rate, and a current of 
from Aveak to medium intensity is made through the prepara- 
tion, the first reaction of all the specimens is to stop their 
forward motion, turn towards the kathode, and start crawling 
very slowly towards this pole. The orientation towards the 
kathode is at the first trial usually rather precise. The whole 
animal gets squarely into line with the current and moves 
slowly towards this pole. While the current is acting the 
anode end of the body, in this case the posterior end, remains 




+ - 




3 + 



d 

Pjg. 42. — Diagram showing; I lie typical electiofaclic reaction of PI an aria. 
a. Position at the moment of making the current, b, c, and d. Suc- 
cessive phases of the reaction. 

rather strongly contracted, presenting the same appearance 
as when mechanically stimulated. Movement occurs only for 
a short time after the current begins acting. The worm soon 
comes to rest, and further stimulation serves only to cause 
contraction of various parts of the body without producing 
any progressive movement. The successive stages of the first 
typical reaction to the constant current are shown in Fig. 42. 
In succeeding experiments on a given individual, and in 
many cases with the very first experiment, the reaction is 



IMOYM.MKNTS, KTC, OF FRKSII-WATER PLANAKIANS. 687 

much less ]')rononiicod. 'I'lie aiiiinal in the transverse position, 
at the moment of makino- tlie current, will sim])lv tnrn the 
anterior ]iart of the body somewhat toward the kathcxh' niid 
tlien stop. Reversal of the current causes the head to swing 
a short distance towards the new kathode, and then stop 
again. The oinentation becomes less and less precise the 
longer the current acts. The position most fre([uently tnken 
by a speciuien after it has been submitted to the action of the 
current for a short time is shown in Fig. 4■^, where it is seen 
that the orientation of even the anterior end is not very 
precise. In all such cases the specimen remains perfectly 
quiet after the first turn towards the kathode until the 
current is reversed or broken. 

The behaviour described is that which is typical for currents 
of medium to fairly Aveak intensities. With verj' weak 
currents no striking effect is produced. With a current 



+ 



Fig. 43. — Diagram showing partial orientation of Plaiiaria to the 
constant, current. 

which is just stong enough to cause a general movement of 
Paramoccium towards the kathode, the only eifect on a 
planarian gliding at right angles to the current is to cause in 
some cases a very slight turn of the head towards the kathode 
at the moment of making. The specimen does not stop the 
gliding movement, and is not forced into any orientation, but 
may give a slight turning reaction, which changes its course 
from one squarely at right angles to the current to one 
turned a little diagonally towards the kathode. In many 
cases such a current produces no effect whatever. With very 
strong currents the planarian stops at the moment of making, 
jerks the anterior end around towards the kathode more or 
less, and then curls up into the form shown in Fig. 44, as a 
result of verv strono- contraction of the ventral lonsjitudinal 




688 RAYMONP PEARFi. 

musculature^ and dies. I have never been able to produce 
disintegration on the anode side Avith any current strength at 
my disposal except in a single case, where disintegration 
began in the region just behind the pharynx in a specimen 
strongly curled up in the way described. 

In case the long axis of the planarian is parallel with the 




V 



Fig. 44. — Diagrammatic side view of a planarian subjecteil to the action 
of a very strong constant current. 

direction of the current, and the head is towards the kathode 
at the moment of making, with a perfectly fresh specimen the 
effect is to cause a cessation of the gliding movement and a 
change to a very slow crawling. The direction of the move- 
ment is not changed. There is a well-marked contraction of 
the anode (posterior) end of the body. The reaction of the 
animal in this position is shown in Fig. 45, J). Yerj weak 
currents have either no effect on a specimen in this position 
or else may cause a very slight contraction of the ventral 
longitudinal fibres mentioned above. 

When the long axis of the body is parallel to the direction 
of the current, and the head is towards the anode at the 




Fig. 45. — Diagram showing tlie elcctrotactic reaction of Plan aria when 
the long axis of the body is in line with tiie current direction, and 
the head is towards tiie katliode. Contracted portions are indicated 
by heavy lines. 

moment of making, the effect of a current of medium intensit}- 
is to cause the gliding movement to stop. At the same time 
there is a very definite contraction of the anode (head) end 
of the body. As the current continues to act the specimen 
begins to squirm about, and very soon gets out of line Avith 
the current. Then the anterior end is turned toAvards the 



MOVEMENTS, ETC, OF FRESH-WATER PLANARIAKS. 689 



kathode slowly, unci this process may be continued until com- 
plete reversal is brought about and the animal comes to lie 
again in line Avith the current, but with the anterior end now 
directed towards the kathode. This reversal into the usual 
orientation is the typical reaction for fresh specimens at the 
first trials of the current ; it is shown in Fig. 46. In case the 
specimens have been under the action of the current for some 
time, there is no reversal of the position. The specimen 



+ 







+ 




J + 



I'lG. 46. — l)iai,Miim bliowiiig the elect rolactic reaction of Plaiiaiia wlien 
the long axis of the body is in line with the current direction, and 
the head is towards the anode. Contracted portions are indicated 
by iieavy lines. 

simply renuiins in the same position and contracts strongly at 
the anode (head) end of the body. 

Strong currents have the same effect as described in the 
preceding experiment. Very weak currents cither have no 
definite effort, or else cause a slight jerking back of the head, 
and turning a little to one side at the moment of making. 

After the animals have become partially paralysed by the 



600 



liAYMOND riOAUL. 



iU'tion ol' till' iMiiTi'iit , llu' iKiliin' ol' tlu- contract ions and 
relaxations of dilTiTi-nt pai-ts of the body can \>c very fU-arly 
scHMi, and since tlicsc arc the most signiiicant features of thi> 
animal's reactions to the electric current, they niay be iK'- 
scribed a little more fully. These reactions for tlie three 
cliii'f positions are shown in I'ig. 47. The essential features 
art" contraction of tlu' anode eml of the body Avlien in line 
with the cui'rent, and convexity on the anode side when at 
right angles. IJesides this there seems to be some slight 
exj)ansion at the kathode end of s]X'cimens in line with the 
curi'ent, but this a])])earance is ne)t constant. Iveversal of the 
current in these paralysed specimens causes contraction at 



/^^- 




+ 



Yic. 4-7. — Diagram bhowiii;;; llic contractions caused by tlie current with 
tlic l)ody in tiic three principal pot-itions. 

the new anode imuI or bending towards the new kathode. On 
breaking the current the contracted portions relax. 

c. Mechanism of the Iveactions. — It will be seen from 
the figures, and the accemnt which has been given ol' tlu' 
n>sponses to the electric current, that there is an apjiarent 
anomaly in the lu'liaviour. The specimen contracts always 
at the anode end of the body, 1)ut a]i])ai'ently not on the 
anoiU' side of the body. 1 believe that the explanation foi- 
this ap]iarent dilferenci' in behaviour is to be found in the 
structure of tlu- animal, and in a pi'culiarity in the action oi 
the constant current which has binm noted in another case. 
When the animal is in line with the current the contraction 



MOVKMKNTS, KTC., ol' KliKSII-WATEIf. l'I-AX.\ l.'l A NS. 001 

(>l>.sei'VC(l at Llio uiiodu uud is, tis shown ])y tlic loiiii taken \>y 
the part reacting-, a contraction oftlio Ion (^-i t luli nal niiisclc- 
fi])rcs_, wliilc I lie (il)rc's of tlic circiihir ;ui<l transverse Kystem 
are relaxed. Jn other words, the ciii-i'ent only alTects those 
fibres which hear a, definite orientation with r(rl:i,li(jn to direc- 
ti(jn oi' its jhnv, vi/. those which ai-e ])ar!iHcl with it. Now it 
has been shown in an eai-lier ])ai-t oi' this ])a|)(M' that in the 
ordinary ne^^ative reaction the tnrnin<^- awjiy i'rointhe stimulus 
is produced l>y a contraction oi' the cii-culai-, transverse, and 
dorso-ventral fibres (})rincipa]ly the cii'cular) on the side 
stimulated. I'^videntiy when the ;iiiiiii;il is at right angles to 
the direction of the flow of the current the only muscle-fibres 
in tiie body \vli(;se longitudimil axes are in line with the 
current are the fibres of the circular and transverse systems. 
Unless it is assumed that the current acts differently in one 
case from in iinothei' theic is no ajtparent i-eiison why, when 
the animal is in the ti"insvei-se ])osition, the; fibi-es which are 
in line with the direction of the curi-ent (jii th(! anode side of 
the body should iK.t conti-act. if the filn-es fullilbng tliese 
conditions as to location and o]-ientati(ni (the circular system) 
do contract, they will cause the anterior end to be turned 
towards the kathode and the anode side to become convex, — 
in other words, produce the actually observed result. The 
fibres of the longitudinal system sliould not be affected, and 
there is no evidence; that they are. This explanation assumes 
that the current produces its effect by directly causing the 
contraction of prf)perly oriented rnuscle-fibres, possibly, or 
even probably, witlujut relation to the stimulation of any 
sense-organs of the aniniiil. Or, to ]nit it in another way, the 
responses according tfj this view might not necessarily be 
reactions of the oi-ganism at all, in tlie sense of being some- 
thing that the animal does after receiving and transfoi-ming 
a stimulus, but are direct effects of the stimulus acting on the 
motor organs. It has doubtless occurred to the reader tliat 
another explanation is possible for these reactions, namely, 
that they are in no way essentially different from what would 
be produced if the animal were given strong mechanical 



692 KAYWOND PEARL. 

stimuli on tliosc parts of the body wliich arc nearest the anode 
in the several positions. In other words, the constant current, 
from the standpoint of the planarian, produces the same 
effect on the anode side or end of the body that a strong- 
mechanical stimulus applied in the same place would. 

Which of these two views is the correct one the planarian 
does not show cleai'ly. Yet there is some inferential evidence 
which makes it seem probable that the first view as to the 
cause of the reaction is the correct one, viz. that the current 
produces direct contractions of muscle-fibres oriented in line 
with its direction. The evidence for this view is as follows : — • 
(1) In the case of specimens which have been for some time 
under the action of the current, and are, as has been 
mentioned, almost completely paralysed, the essential features 
of contraction on the anode side or end can still be pro- 
duced by a fairly weak current. At the same time it takes a 
very strong mechanical stimulus to get any reaction from 
these pieces, indicating that their sense-organs are almost 
completely paralysed, and their general sensitivity gone. If 
the current acts merely as a stimulus qualitatively like 
others which produce the same reactions, it is not apparent 
why it should be effective in weak intensities when another 
stimulus fails in strong intensity. If it acts directly on the 
muscles we should expect that it would be capable of pro- 
ducing an effect after the general sensory functions had been 
lost. (2) The contractions produced by the current are 
sharply localised, i. e. they involve only a certain definite 
part of the body whether the current is strong or weak 
(within certain limits) ; whereas mechanical stimuli applied 
to the same places with an intensity sufficient to cause the 
same definitive reaction will also cause a marked general 
response of the whole organism. This is just what would ])c 
expected if the current affects only the muscles oriented in 
line with it and lying at the anode pole of the worm, (o) 
By analogy with other forms — for example, the Protozoa — it 
Avould be expected that the curreiit would |)roduce some 
other effect than that of an ordinary stimulus applied at the 



M0VE.MENT8, ErC, OK FRKSll-WATKR ri,ANARIANS. 693 

same point. In tlie case of the Ini'ui^oria the current causes 
an entirely different reaction from tliat pi'oduced by any 
other known stimulus. 

For these reasons, then, I am inclined to think that in the 
case of the flat-woinn the current affects certain definitely 
oriented nniscle-fihres directly, and by this means ])roduces in 
the main the characteristic reactions. That the current does 
not also stimulate the sense-organs, and so act like other 
stimuli applied to the same places, I am not })repared to say, 
but it seems probable that the phenomena observed are not 
primarily caused by such action. 

It has been brought out l)y inference that the cilia play 
no part in the electrntactic reaction of planarians. This is 
the true state of the case. The current in any intensity 
sufficient to cause the definite reactions stops immediately, so 
far as I have been able to observe, all ciliary movement. The 
evidence for this is twofold. First, all gliding movement 
stops in effective currents ; and second, by direct observation 
of specimens crawling ventral side up on the surface film no 
ciliary currents can be observed while the electric current 
acts. This result is of interest in connection with the 
reactions of the rhabdocoele Steno stoma leu cops, O. 
Schm. This form, which normally moves freely through the 
water by the activity of its cilia, reacts to the electric current 
in essentially the same way as do the Infusoria (cf. Pearl, : 00), 
That is to say, the cilia on the kathode half of the body take 
a reverse position when the current is made, and their effective 
stroke is towards the anterior end. The different relations of 
the cilia in different positions of the body are shown in 
Fig. 48. This relation of the ciliary beat, coupled with the 
form of the body, causes, as a uiechanical necessity (cf. 
Ludlolf, ^95), the animal to orient with the anterior end 
towards the kathode. This method of reaction of S t e n o s t o m a 
I worked out by precisely the same methods as I used in a 
])revious study of the electrotaxis of the Infusoria (:0()). 
This reversal of the position of the cilia as a result of the 
action of the current has hitherto been observed only in 



694 



EAYMOND PKAHL. 



the Infusoria, and to fiiul the samo tiling in a multicollular 
organism is a matter of considerable interest. It is out- 
side the scope of the present paper to discuss the relation 
of this result to current theories of electrotaxis, as I hope 
to be able to do in a later paper, but it may be said that 
this furnishes another strong piece of evidence that in the 
case of these loAver organisms the current does not cause the 
observed reactions in any way comparable to that in which a 
mechanical stimulus causes a reaction, i. e. by furnishing a 
certain "sensation." On the contrary, the current acts as 
a physical force on a structure organised in a certain way. 
Experiments on the electrotactic reaction of cut pieces of 



lfy^^yyy^^--y^^A~,-~^ .^^^^^y^ ^^, 







Fig. 4S. — Diagram sliowiii^ (he electrotactic reaction of the rhabdocele, 
St en OS to ma leucops, O. Schm. 

planarians have been tried in considerable numbers, but Avith, 
on the whole, unsatisfactory results. Anterior pieces result- 
ing from transvei'se cuts are the only ones from which I have 
been able to obtain any constant results. Such pieces react 
like the normal animal in every way. Posterior pieces from 
transverse cuts show the contractions on the anode side and 
ends in a slight degree, but there is no constant production 
of orientation. Specimens slit longitudinally in the middle 
line from the posterior end nearly to the head react 
essentially like a normal specimen, although much more 
weakly. I have observed in one case fairly precise orienta- 
tions of such a specimen. From specimens slit longitudinally 



:\[OVE.MI'JNTS, KTC, OF FRI'JSH-WATPIIJ PLANAlv'IAN.S. ()95 

ill the iniddlo line from in front backwards 1 have never been 
able to obtain any definite results. Tliey simply s(|uirm 
about in an aimless way for a mcjnu'ut when ihc eunent is 
made and then become quiets and remain so while the current 
passes. 'J'he direction in which tlie current is flowing makes 
no diifereiice in their behaviour. All operated specimens 
become very quickly paralysed by the cun-ent. 

d. Summary. — The constant current very quickly para- 
lyses planarians. Its specific effect is to cause a contraction 
of the anode side oi- end of the body. This produces in the 
case of fresh specimens a well defined orientation, with the 
anterior end towards the kathode. All prog-ressive move- 
ment after the making of an effective current is by the crawl- 
ing method, the cilia being stopped or very greatly slowed 
in their beat. The electrotactic reaction, so far as the attain- 
ment of orientation is concerned, is essentially the same as 
the negative reaction to mechanical stimuli. In the rhab- 
docccle Stenostoma leucops there is found to occur a 
reversal of the cilia on the kathode half of the body, such as 
occurs in the case of the Infusoria. 

V. Keaction to Desiccation. 

A series of experiments Avas performed to determine the 
reactions of the aninud on drying. This is an environmental 
condition Avhicli planarians ])robably have had to meet with 
relative fre(|uency in the course of their history as a species, 
and it is a matter of interest to determine whether they have 
any method of reacting which protects them from it. 

Experiments were flrst performed in the following- 
manner : — Specimens were taken from the aquarium dish on 
the point of a scalpel or a spatula, and lightly touched to a 
filter-paper for a moment to remove any adherent water, and 
then laid upon a dry surface — either glass or ])aper. The 
behaviour was usually as follows : — The worm would curl up 
closely and thrust the head under the body, as shown in 
Fig. 49. The purj)()se of their behaviour seems to be to get 
the body into as small space as [jossible, and especially to keep 



696 RAYMOND PEARL. 

the head from drying. At fairly frequent intervals the 
animal straightens out and extends the head in front as far 
as possible^ and makes "feeling'^ movements. It is then 
withdrawn, and the animal curls up again. After the drying 
has proceeded for some time the inost characteristic feature 
of the whole reaction appears. This is a lengthening of the 
posterior part of the body to its fullest extent. The posterior 
end then attaches itself to the surface, and strong waves of 
contraction, like those in the crawling movement, pass over 
the body from the posterior end forward. No progressive 
movement is made, but backward crawling is evidently 
attempted, and is only prevented by the dry surface Avhich 
the animal is on. There may be considerable variation in the 
first part of the reaction with regard to the curling up ; this 
may appear or may not, but the attempted backward crawl- 




¥iG. -t'J. — Diagram showing the reaction of Planaria to desiccation. 

ing movement of the posterior part of the body I have found 
to be a constant feature in the experiments which I have 
performed. When the dorsal surface of the worm becomes 
dry all movement ceases. If quickly put back into the water 
the worm will usually recover completely, even though all 
movement has ceased in the air. 

If the Avorm is put on a slide in the centre of a small area 
which has been wet, but on which there is no standing water, 
it will squirm about and extend the head frequently, as in 
the last experiment. If the head goes outside the wet area 
it is very quickly jerked back, and the specimen gives the 
negative reaction, i. e. turns away from the side stimulated. 
The attempted backward crawling occurs in this case just as 
in the others, a short time before the dorsal surface dries off. 

It is to be noted that there is never any actual progressive 



MOVEMENTS, KTO., OF TUKsn-WATKl^ rLANATHANS. G97 

movement of a specimen in tlie nir. If a s])ecinien is placed 
on very Avet filter-])a])er it is not ahle to jtroo-ress nnless 
water is kept constantly clroppino- on it from above, so that it 
is at any time snrronnded by a laj^er of water. On account 
of this lack of ability to move when out of water, there is no 
true hydrotaxis in the sense of movement towards Avater. 

As has been mentioned before, specimens placed on a dry 
surface dorsal side down do not show the rightino' reaction. 

To sum up, it is found that planarians, when removed 
from the water and subjected to a ])rocess of tlryiny, are 
unable to make progressive movements. At a certain stage 
in the drying* process they attempt to crawl backwards — a 
form of movement which, under certain circumstances, might 
get the animal back into water. On meeting a dry surface 
with the anterior end the animals give a well-marked negative 
reaction. The animal does not give the righting reaction on 
being inverted on a dry surface. 

On the whole, the general behaviour when subjected to 
drying is purposeful ; that is, it would tend to prevent the 
animal ever becoming dried up under natural conditions. 
There is nothing in the behaviour of planarians to indicate 
how the change from aquatic to terrestrial life could be 
brought about. T^he fresh-Avater Triclads, so far as I have 
observed them, never leave the water and crawl u}) into the 
air above the surface film as some other forms do. 

VI. Rheotaxis. 

A large number of experiments were performed early in 
the course of the work with various sorts of devices to deter- 
mine whether the animal shoAved any distinct reaction to 
currents in the Avater, but without success. Streams of Avater 
from a pipette, currents made by filling the tube of the 
diffusion apparatus described above (pp. 661, 662) with water 
and bloAving into it, and other methods gave no results. If the 
currents Avere made Avith sufficient force to threaten dislods"- 
ment of the animal from its hold on the bottom it Avould stop 
movino- and contract longitudinal) v, and thus attach itself 



698 I^AYMOND rEARL. 

more firmly to the substrate. Weaker currents caused no 
effect wliatever. I was inclined to believe that the lono-jtu- 
dinal contraction and the gripping" of the bottom Avere tlie 
only rheotactic reactions which the organism exhibited. It 
was found later, however, that there was a very precise 
rheotactic reaction of a different character. In the course 
of the experiments on reactions to localised chemical stimuli 
by the capillary tube method, it was discovered that by using 
a tube with a relatively large opening (from j to 4 mm. in 
diameter) and letting the ordinarj^ tap-water in which the 
animals were flow out of it, by its own weight, a current of 
just the right intensity to cause a positive reaction could be 
produced. The animals would turn very sharply towards the 
source of such a current, the reaction being evidently the 
same as that given to other weak stimuli (chemical and 
mechanical). This reaction is localised in the same way as 
the usual positive reaction. It is given only when the current 
is directed against the head or anterior part of the body. 

It is thus seen that the planarian is positively rheotactic to 
very weak currents, the form of the reaction being precisely 
the same as that given to other weak stimuli. It seems very 
doubtful if this reaction is of any impoi'tance in the normal 
activity of the animal. 

G. General Summary and Discussion of Results. 

As was stated earlier in the paper, the problem witli which 
this study deals is the analysis of the beliaviour of the 
common fresh-water planarian. The movements and reac- 
tions to all the more important stimuli, with the exception of 
lio-ht and heat, have been described and analysed into their 
component factors in the body of the paper. It is believed 
that it is of the greatest importance to have as complete and 
detailed an account of the various activities as possible, and 
as a consequence full details have been given in the case of 
each subject treated. Since this method of treatment 
necessarily makes the account of considerable length, it has 
a tendency to obscure the general and significant results in a 



MOVEMENTS, ETC., OF FEESn-WATKK PLANAKTANS. G99 

mass of detail. It is desirable, then, to state clearly at the 
end the impoi'tant general facts which have been bronoht ont 
by this study, and to discuss to some extent their significance. 
In this place I shall state the results in a categorical manner, 
making no attempt to indicate the evidence on which the 
conclusions are based. This will avoid needless repetition. 

1. The locomotor movements of PI an aria are of two sorts, 
gliding and crawling. The gliding movement is produced 
by the beating of the cilia on the ventral surface of the 
organism. It is by far the most usual method of locomotion. 
For its production it is necessary that thei'e be a layer of 
sticky, mucous slime between the ventral surface of the body 
and the substrate. In this slimy secretion the cilia beat and 
so propel the animal (cf. pp. 544 and 545). The organism 
never moves freely through the water without some sort of 
mechanical support. The rate of the gliding is changed by 
the action of various agents, such as light, chemicals, elec- 
tricity, etc. Its direction is always forward. 

The crawling movement is produced by strong longitudinal 
waves of muscular contraction passing over the body from 
the anterior to the posterior end. It is more rapid in rate 
than the gliding. It appears only after strong stimulation 
of the organism, and its purpose is evidently to get the 
animal quickly away from harmful stinuili. Its direction 
may be either forward or backward. 

Periods of movement alternate with periods of rest in the 
course of the animaPs daily activity. When at rest the flat- 
worm is in a condition of relaxation and generally lowered 
tonus, corresponding to the condition of a higher organism 
in sleep. The causes which induce the coming to rest are — 
(a) a more or less fatigued condition of the organism. This 
is the primary cause ; without it the other causes are ineffec- 
tive. {!)) A relatively low intensity of light, (c) Koughness of 
tlie substrate. This brings the body into a position such that 
its different parts form angles with one another, and causes 
the animal to couu' to rest as the result of a reaction wiiich 
I have called goniutaxis (p. 5G2). (d) Certain chemical con- 



700 BAYMONP riOAKL. 

ditions. As a result of the action of some one oi" all of these 
above-meutioiied factors^ collections or groups of ])lanarians 
are frequently formed. 

Planarians which have been injured by operative procedure 
move comparatively little during the course of regeneration, 
thus showing a sort of regulation or correlation between 
behaviour and morphogenetic processes (pp. 573, 574). 

2. There are two principal qualitatively different reactions 
to stimuli, the positive and negative reactions. 

The negative reaction is given in response to strong 
unilateral stimulation of the anterior portion of the body. 
It consists essentially in a turning of the head away from 
the side stimulated. It is brought about by the extension of 
the body on the side stimulated. This extension is produced 
by a contraction of the circular, dorso-ventral, and transverse 
systems of muscle-fibres. The purpose of the negative 
reaction is evidently to get the organism aAvay from harmful 
stimuli. 

The positive reaction is given only in response to weak uni- 
lateral stimulation of the anterior portion of the bod}-. It is 
essentially a turning of the head towards tiie source of the 
stimulus. This reaction is one of considerable precision, 
bringing the anterior end into such a position that it points in 
most cases exactly towards the source of the stimulus. The 
turning is brought about by the contraction of the longi- 
tudinal muscle-fibres of the side stimulated. The evident 
purpose of the positive reaction is to get the animal into 
regions of beneficial stimuli. 

3. Whether the negative or the positive reaction shall be 
given in response to a particular stinndus depends primarily 
on the intensity of the stimulus, and secondarily on its loca- 
tion. Neither reaction is given unless some part of the body 
in front of the pharyngeal region is stimulated. The negative 
reaction is given only in response to stimuli above a 
certain intensity (strong stimuli). This relation between 
intensity of stimulus and form of reaction holds for both 
mechanical and chemical stimuli. 



MOVKMI'INTS, ETC., OF FRESH-WATER PLANARFANS. 701 

4. The reactions of Planaria to a variety of chemicals, in- 
cluding- representatives of several of the most important 
chemical groups, were studied. It was found that to a weak 
solution of any substance, regardless of its chemical composi- 
tion, the organism gave a positive reaction identical with the 
positive reaction to mechanical stimuli. To strong solutions 
of the same substances (with a single exception, see p. 657) 
the organisms responded by a negative reaction identical with 
that caused by strong mechanical stinmli. 

Planaria does not orient itself to a diffusing chemical in 
such a way that the longitudinal axis of the body is parallel 
to the lines of diffusing ions. Its reactions to chemicals are 
motor reflexes identical with those to mechanical stimuli. The 
positive reaction is an orienting reaction in the sense that it 
directs the anterior end of the body towards the source of the 
stimulus with considerable precision, but it does not bring 
about an orientation of the sort defined above. 

5. Several important features in the normal behaviour of 
the flat-worm are found upon analysis to have their explana- 
tion in the positive and negative reactions to mechanical and 
chemical stimuli. 

The method by which the organism gets its food is simply 
a special case of the positive reaction. From substances 
which serve as food for the planarians, various juices diffuse 
into the surrounding water. When the planarian meets any 
of these diffusing substances it gives the positive reaction, — 
that is, turns in the direction from which the stimulus comes. 
The food substance acts as a weak chemical stimulus, to 
which the animal reacts in the same way as to all other weak 
chemicals. 

The direction of the planarian's movement, and its behaviour 
with reference to obstacles in its path, are usually deter- 
mined by its reactions to mechanical stinuili. 

The behaviour of the organism with reference to the 
surface film is determined by its reactions to mechanical 
stimuli. 

0. Strong stinmlation — either mechanical or chemical — of 

VOL. 46, PAKT 4. — NKW SERIES. Z Z 



702 RAYMOND PEARL. 

the posterior portions of the body induces the crawling move- 
ment. This is to be regarded as the specific reaction of this 
portion of the body. Weak stimidation of the same region 
causes local contraction at the point stimulated in the case of 
mechanical stimuli, while weak chemical stimuli applied to 
this res'ion are ineffective. 

7. The ventral surface of the body of PI an aria is .strongly 
positively thigniotactic, and the dorsal surface is negatively 
thigmotactic. 

8. When the organism is placed in an inverted position it 
performs the righting reaction. This reaction consists in a 
turning of successive parts of the body about the longitudinal 
axis through 180°. During the process the animal takes the 
form of a spiral. The anterior end is brought into the up- 
right position first. On analysis the righting reaction is 
found to be a special case of the reaction to strong stimuli 
(the negative reaction) . It is brought about by an extension 
of one side of the body^ while the other side maintains its 
original length (pp. 676 — 679). The reaction is given when- 
ever the ventral surface is removed from a solid or the surface 
film of the water. 

9. To the constant electric current Planaria reacts by 
turning the anterior end towards the kathode. Complete 
orientation and movement towards the kathode may occur. 
Tlie turning towards the kathode is brought about by an 
extension of the anode side of the body. The current causes 
a contraction of muscular elements whose long axes are 
parallel to the direction of the current (pp. 690 — 693). The 
current very quickly paralyses planarians on which it acts. 

The rhabdocoole Steno stoma leucops orients to the 
current with the anterior end towards the kathode, and 
moves towards this pole. This orientation is brought about 
by changes in the positions and consequent effective beat of 
the cilia, exactly like those which occur in the case of the 
ciliate Infusoria. Cilia, on the portions of the body directed 
towards the kathode pole, take on reversed positions. 

10. All the normal reactions to stimuli are of the nature of 



MOVEMENTS, KTC, OF FKESH-WATER PLANAKIANS. 703 

reflexes, more or less complex. What the animal will do 
after a given stimulus, or in a given situation, can be predicted 
with reasonable certainty. There is, however, some variation 
in the behaviour, depending on the physiological or tonic 
condition of the individual at the time of stimulation. Thus 
a stinuilus sufficiently weak to induce the positive reaction in 
one specimen may cause the negative reaction in another ; or 
at different times the same individual may show different re- 
actions — either the positive or negative' — to the same stimulus. 

11. Psychological Position of Planaria. — The objec- 
tive psychological position of any organism is evidently deter- 
mined by the relative simplicity or complexity of what it 
does. With a view of determining what the position of 
Planaria in the psychological scale is, it may be well to 
make a catalogue of the things which it does in the course 
of its ordinary existence. 

The animal performs the following acts : 

a. It moves progressively by two methods, a ciliary motion 
and a muscular motion. 

h. It turns, by a complex of simple reflex acts, towards all 
weak stimuli investigated. 

c. It turns, by another set of simple reflex acts, away from 
all strons: stimuli investisj'ated. 

d. It comes to rest in certain definite environmental 
situations. 

e. When stimulated in a certain way it extends the pharynx 
and feeds. 

/. Wlien its ventral surface is removed from contact with 
a solid body (or the surface film), a reflex of essentially the 
same character as that of c brings this surface again into 
contact with the solid. 

From these essential factors is composed a behaviour 
whose complexity one has only to study to realise. 

The behaviour is thus seen to be, in the main, what may 
be characterised as reflex. It is very simple to say that an 
animal's activity is composed of a series of invariable reflex 
acts in response to stimuli, but I doubt whether the full 



704 EAYMOND PKAUL. 

significance of such a condition is always realised. It implies 
that the animal as an individual " does " nothing in the sense 
that a man " does " things. It is moved about from place to 
place by its locomotor organs; it is put into certain definite 
and invariable relations to its surroundings by its reflex 
mechanisms. Considered as a wholes such an organism is a 
sort of shell to hold a series of mechanisms, each of which is 
independently capable of doing a certain thing, and in the 
doing produces some effect on the shell as a whole. We may 
perhaps got a clearer picture of what such a reflex existence 
means by considering for a moment what would be the effect 
if all a man^s activities were composed of invariable reflexes, 
to be set off by the appropriate stimuli. Under such circum- 
stances, whenever a man saw or smelled food he would have 
to go to it and eat it. Whenever anything touched him he 
would have to move in a new direction very closely related 
to the position of the object which touched him. Whenever 
he touched water he would have to take a bath^ or perhaps 
drink till he could hold no more. During the day he would 
have to move always in a definite direction with reference to 
the sun, and so on ad infinitum. All he did would be 
definitely fixed and, in a sense, predetermined by the things 
about him. 

It is apparent that the behaviour of Planaria is not thus 
entirely and purely reflex, because there is a certain amount 
of variation in it. As has been brought out in several places 
in the body of the paper, and in paragraph 10 of these 
conclusions, this variation in the behaviour is the result of 
the physiological condition of the individual. To put this in 
a more concrete form, we may say that a fatigued animal 
or an animal in a state of great excitation does not always 
react to a certain stimulus by the same set of reflexes as that 
by which a normal animal would react. Furthermore, there 
is a variation in the intensity of the negative reaction 
dependent upon the intensity of the stimulus producing it. 

Another point in which the reactions of Planaria differ 
from what would obtain in the case of an organism whose 



MOVKMRNTS, ETC., OF FRKSII-WATKIJ I'LANAUFANS. 705 

behaviour was composed of invavial)le reflexes is foniicl in tlie 
behaviour foHowing repeated stroug stimuli applied to tlie 
anterior end (vide pp. 580, 581). In this case the organism 
shows an evident modihability in reaction, for after giving 
for some time the ordinary negative reaction, and not thereby 
getting away from the stimulus, it finally turns directly 
towards the source of the stimulus. Again, in the righting 
reactions of pieces of the body we see entirely new forms of 
reaction appearing (pp. 680 — 683). 

In order to give a concrete idea of the psychological 
position of Planaria it may be well to present in parallel 
columns the principal factors which make for simplicity in 
the behaviour on the one hand, and for complexity on the 
other hand. 

r actors which tend to make Factors which tend to make 
tlie Behaviour Simple. the Behaviour Complex. 

A. Essential reflex character at the A'. Comparatively large number of 

basis of all the reactions. qualitatively different general 

reactions, 

B. General lack of modifiability of B'. Marked qualitatively different re- 

reactions, actions to differing intensities of 

stimulus. 

C. Comparatively small number of C. Definite relations of reactions to 

qualitatively different reflexes location of stimulus, 

composing the general reac- 
tions. 

D'. Rather close dependence of reac- 
tions on the physiological condi- 
tion of tlie individual. This brings 
about variation in the reactions. 

The behaviour of Planaria is evidently much more com- 
plex than that of the Infusoria, as described by Jennings 
(loc. cit.). In the case of the Infusoria, all the factors A', B', 
C, D', which make the behaviour of Planaria so complicated, 
are nearly or quite absent ; and in respect to C these organ- 
isms are at a much lower stage than Planaria. The 
Infusoria have practically but one purely reflex reaction to 
nearly all stimuli, and this reaction is not localised with 



706 RAYMOND PEARL. 

reference to the location of the stimuhis. Again, the Infu- 
soria do not show qualitatively different reactions to differing 
intensities of stimuli, as does PI an aria to a marked degree. 
We thus see that Planaria stands considerably higher in 
the psychological scale than the Infusoria, and that the 
development is taking place along two main lines : (a) the 
higher organism reacts differentially with reference to the 
location and intensity of the stimulus ; and (b) the physio- 
logical balance in the higher organism is much more deli- 
cately adjusted than in the lower, and as a consequence we 
see much more variation in the physiological condition. 
These variations in the ]3hysiological condition bring about 
variability in the reactions. 

In the case of the ctenophore Mnemiopsis Leidyi we 
have an intermediate stage between the Infusoria and 
Planaria, Here the animal reacts with reference to the 
position, but not the intensity of the stimulus. This condi- 
tion, in which an organism reacts with relation to the position 
of a stimidus, and not to its intensity, must be for the indi- 
vidual a precarious one, because the animal must either go 
towards or away from all stimuli alike, whether good or 
harmful. Chances are theoretically equal that after each 
stimulus it may get a toothsome morsel of food, or, on the 
contrary, serve in that capacity itself. Further development 
beyond the point in the behaviour series where Planaria 
stands must be in the line of further differential reactions 
with reference to quality of stimulus. A beginning along 
this line is made by the planarian, and the process is carried 
a step farther in the case of G onion em us, as recently 
described by Yerkes (loc. cit.). 

12, llelation of Behaviour and Structure. — The reac- 
tions of organisms are evidently, in any case, very closely de- 
pendent on the structural relations of the given organism, and 
on the conditions under which it lives, i, e, its environment 
in the broadest sense. Thus we find the asymmetrical Infu- 
soria, which live freely in the water and move about by means 
of cilia, all reacting in the same way, and the determinative 



MOVEMENTS, KTC, OF FRESH-WATER PLANAIflANS. 707 

factor in the reaction is the asymmetry of the body (cf. 
Jennings, :00). Now Jennings has further founds that 
certain rotifers, Avhich live freely in the water and move 
about by the activity of cilia in a similar way, and further- 
more are asymmetrical in fundamentally the same way that 
the Infusoria are, react in essentially the same manner as do 
the Infusoria. Similarly, I believe that the general reactions 
method of the planarians may be found to be in the main the 
method by which all organisms presenting the same general 
structural relations and mode of life react. Only one 
example on which this conviction is based may be given 
here. In the case of such fresh-water molluscs as Physa it 
is apparent that the actual locomotor and sensory organisa- 
tion is symmetrical in form, and furthermore these forms live 
in fresh Avater on the surface of solid bodies just as do 
planarians. Now I have found, in a series of observations 
not yet published, that in the case of several of these molluscs 
the fundamental scheme of reaction is like that in the 
planarian. They react in the same way with reference to 
the location and intensity of the stimulus, and these are the 
fundamental things. In fact, the general behaviour is 
strikingly alike in the two widely separated groups. 

13. Purposive Character of Reflexes.— A fact which 
is strongly imjiressed on one working on the behaviour of an 
organism whose activities are largely reflex is the purposive 
character of these reflexes. They are so adjusted that in the 
long run they keep the animal out of danger, and get it into 
favourable conditions. In the flat-worm these two things are 
very well done in general by the negative and positive 
reactions. Of these two reactions it is easy to see tliat the 
positive is the more highly developed, in particular in the 
fact that it is much more precisely localised with reference to 
the position of the stimulus. We can see a reason for this in 
the fact that under the conditions of the planarian's life the 

' Complete observations not yet published. For preliminary account see 
' Science,' N. S., vol. xv, pp. 521 and 525 ; and Jennings, : 01, in bibliography 
at the end of this paper. 



708 RAYMOND PEARL. 

getting of food is of far more importance in tlie strnggle for 
existence than the avoidance of danger. This point has, 
however, been discussed earlier in the paper, and need not 
detain us here. The real problem is presented in the attempt 
to discover how any of the purposive reflex acts in the 
organisms arose. I see no reason for denying that man}- of 
them — such as, for example, the positive reaction which gets 
the animal its food — were developed by natural selection. 
There are other evidently purposeful reactions, however, with 
whose development it hardly seems as if natural selection 
could have had anything to do, since they cannot themselves 
be of selective value. This point has been well brought out 
in a recent paper by Morgan (: 02, p. 281). I think a pos- 
sible explanation of some of these may be found in their 
analysis into component factors, when it may appear that 
only a very few simple reflexes had to be formed by natural 
selection, and then all the reactions are built up from these. 
An example will make my meaning clearer. In the righting 
reaction of the planarian we have a fairly complex reaction 
which is evidently immediately jDurposeful. Yet we find on 
analysis that this reaction is at bottom nothing but a slight 
modification of the ordinary negative reaction, which might 
very well have been developed by natural selection. And 
thus it is with other reactions and pieces of behaviour. They 
are for the most part built up from a very few simple 
purposive reflexes. If we can get them subdivided and 
spread out, as it were, so that we can see what goes to 
compose them, we may find that our problem has diminished 
very much, and we shall have to deal with only a few factors 
where before there appeared to be many. 

A difficult problem in purposeful behaviour presents itself 
when we find that new methods of reaction appear at once if 
the usual reaction is prevented. The best examples of this 
are found in the righting reaction of cut pieces of planarians. 
Here we find pieces of the body, in which the noannal 
mechanism of the reaction has been destroyed, immediately 
reaching a certain end (the righting) by a method differing 



MOVRMKNTS, ETC., OF FKESH-WATER I'LANARIANS. 709 

entirely from any tliat planarians ever nsed before to attain 
the same end, so far as we have evidence. Tliese ]ihenomena 
have a eonsi(U'ral)le reseml)liince to sucli phciKHiiena as tlie 
well-known reo'eneration of the lens from the iris in some 
Amphiljia. It is not easy to see how snch behavionr comes 
about, and iiaturiil seh'ction helps ns very little. The matter 
belono's apparently to the same class of ])henomcnii ;i's 
morpliological regulations, and probably h:is ultimately the 
same explanation. What this explanation is we do not know. 

14. Functions of the Nervous System. — The most im- 
portant function of the brain is the preservation of the tonus 
of the organism. After its removal the general tonus rapidly 
diminishes, and on this account the positive reaction — which 
depends rather closely on the physiological condition — c:in 
be obtained only with great ditticulty in such deca])itated 
specimens. There is no evidence of the presence of special 
centres in the brain. The nervous system, as a Avhole, has 
its main function in the rapid conduction of impulses. 

15. Subjective Psychic Attributes. — One of the 
principal questions which forever recurs with regard to work 
on animal behaviour is, does the animal possess conscious- 
ness ? Now although it has been shown what the component 
parts of the activities of the planarian are, yet it cannot be 
said, as it seems to me, that the planarian does not, or, on the 
other hand, that it does, possess consciousness. All that any 
such an organism ever has done in the past, or ever will do 
in the future, cannot tell us whether it was conscious in the 
doing or not. Any " objective criterion " of consciousness 
does not exist. Furthermore, whether consciousness is or is 
not present in any given case is not, in any event, the greatest 
concern of the physiologist, who rests content with the objec- 
tive explanation of how results are brought about, regardless 
of what the animal is thinking about the matter. On this 
subject Claparede (: 01, p. 24), in concluding an interesting 
and valuable discussion, has said, " A la question ; les 
animaux sont-ils conscients ? la physiologic — et meme la 
psychologic en tant que cette science est explicative — doivent 



710 KAYMOND PEARL. 

done repoudi'e iion seulementj ' Je I'ignore/ mais encore, 
' Pen m'iniporte ' ! " With this standpoint I am in thorough 
accord. 

H. List of Literature. 

Bakdeen, C. R. : 01. — "On the Physiology of llie Planaria niaculata, 

with Especial Reference to the Piienonieiia of Regeneration," ' Anier. 

Jouin. Plivsiol.,' vol. v, pp. 1 — 55. 
Bardeen, C. R. : 01, r/. — "The Function of the Brain in Planaria 

maculata," ibid., vol. v, pp. 175 — 179. 
Chichkoff, G. D. '02. — " Recherches sur les Deiidrocoeles d'eau douce 

(Triclades)," ' Arcli. de Biol.,' t. xii, pp. 435 — 568, pis. xv — xx. 
Child, C. M. : 01. — "Tlie Habits and Natural History of Stichostemma," 

'Amer. Nat.,' vol. xxxv, pp. 975—1006. 
Claparede, E. : 01. — " Les Animaux soiit-ils Conscients ? " ' Rev. Philos.,' 

t. li, pp. 481—498. 
DuYNE, J. VAN. '96. — " Ueber die Heteroniorphose bei Phuiarien,"' ' Arch. 

f. d. ges. Physiol.,' Bd. Ixiv, pp. 569 — 574, pi. x. 
Frandsen, P. :01. — "Studies on the Reactions of Limax maximusto 

Directive Stimuli," ' Proc. Anier. Acad. Arts and Sci.,' vol. xxxvii, pp. 

185-227. 
Gamble, F. W. '93. — " Contributions to a Knowledge of British Marine 

Turbellaria," 'Quart. Journ. Micr. Sci.,' vol. xxxiv, ])p. 433 — 528, 

pis. xxxix — xli. 
Hesse, R. '97. — "Untersuchungen iiber die Organe der Lichtempfindung 

bei niederen Thieren : II, Die Augen der Plathelniinthen, insondcrhcit 

der tricladen Turbellarien," 'Zeitschr. f, wiss. Zool.,' Bd. Ixii, pp. 527 — 

582, Taf. xxvii and xxviii. 
Jennings, II. S. '97. — "Studies on Reactions to Stimuli in Unicellular 

Organisms: I, Reactions to Chemical, Osmotic, and Mechanical Stimuli 

in the Ciliate Infusoria," 'Journ. Physiol.,' vol, xxi, pp. 259 — 322. 
Jennings, H. S. '99. — "Sludies on Reactions to Stimuli in Unicellular 

Organisms : II, The Mechanism of the Motor Reactions of Paramecium," 

' Amer. Journ. Physiol.,' vol. ii, pp. 311 — 341. 
Jennings, H, S. '99, a. — " Studies on Reactions to Stimuli in Unicellular 

Organisms: III, Reactions to Localised Stimuli in Spiiostomum and 

Stentor," ' Amer. Nat.,' vol. xxxiii, pp. 373—389. 
Jennings, II. S. '99, Zi. — "Studies on Reactions to Stimuli in Unicellular 

Organisms : IV, Laws of Chemotaxis in Paramecium," 'Amer. Journ. 

Physiol.,' vol. ii, pp. 355—379. 



MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 711 

Jennings, 11. S. '99, c. — "Tlie PsyclioloQ:y of a Protozoan," ' Ainer. Joiirii. 
Psychol.,' vol. X, pp. 503 — 515. 

Jknnings, II. S. : 00. — "Studies on Reactions to Stimuli in Unicellular 
Orpjanisms : V, On the Movements and Motor Reflexes of the Flagellata 
and Ciliata," ' Amer. Journ. Physiol.,' vol. iii, pp. 229 — 200. 

Jennings, H. S. :00, r^ — ■" Tlie Behavi')nr of Unicellular Organisms," 
' Woods HoU Lectures for 1899,' pp. 93-112. 

Jennings, II. S. : 00, i. — " Studies on Reactions to Stimuli in Unicellular 
Organisms: VI, On the Reactions of Chilomonas to Organic Acids," 
'Amer. Journ. Piiysiol.,' vol. iii, p|>. 397 — 403. 

Jennings, II. S. : 00, c. — " Reactions of Infusoria to Chemicals : a Criti- 
cism," ' Amer. Nat.,' vol. xxxiv, pp. 259—205. 

Jennings, H. S. :01. — "On the Significance of the Spiral Swimming of 
Organisms," ibid., vol. xxxv, pp. 369 — 378. 

Jennings, II. S., and Moore, E. M. : 02. — " Studies on Reactions to Stimuli 
in. Unicellular Organisms : VIII, On the Reactions of Infusoria to 
Carbonic and other Acids, with Especial Reference to the Cause of the 
Gatherings spontaneously formed," 'Amer. Journ. Physiol.,' vol. vi, 
pp. 233—250. 

JiJiMA, J. '84. — " Untersuchungen iiber den Bau und die Entwicklungs- 
geschichte der Siisswasser Dendrocoelen (Tricladen)," ' Zeitschr. f. wiss. 
Zool.,' Bd. xl, pp. 359— 4G4, Taf. xx— xxiii. 

Jordan, H. : 01. — "Die Physiologic der Locomotion bei Aplysia lima- 
cina," ' Inaug. Diss. Bonn,' 1901 (Druck von Oldenbourg, Miinchen), 
p. 51, Taf. i. Also in ' Zeitschr. f. Biol,' Bd. xli, pp. 196—238, Taf. ii. 

Kennel, J. vox. 'SS. — "Untersuchungen an neuen Turbcllarien," 'Zool. 
Jahrb.,' Abth. Anat. und Ont., Bd. iii, pp. 447 — 486, Taf. xviii, xix. 

Lang, A. '81. — "Untersuchungen znr vergleichenden Anatomic und Ilis- 
tologie des Nervensystems der Plathelminthen : IV, Das Nervensystem 
der Tricladen," 'Mitth. a. d. Zool. Stat. Neapcl,' ]?d. iii, pp. 53 — 70, 
Taf. v, vi. 

Lang, A. '81, a. — " Untersuchungen znr vergleichenden Anatomic und 
Histologic des Nervensystems der Plathelminthen : V, Vergleichende 
Anatomic des Nervensystems der Pialhelminthen," ' Mitth. a. d. Zool. 
Stat. Neapel,' Bd. iii, pp. 76—95. 

Lang, A, '84. — " Die Polycladen (Seeplanarien) des Golfes von Neapel und 
der angrenzende Meeresabschnitte," ' Fauna u. Flora d. Golfes von 
Neapel,' XI Monographic (Leipzig, Engelmann), pp. x and 688, 
Taf. xxxix. 



712 BAYMOND PEARL. 

Lehnert, G. II. '91. — " I5eobacIiliuigen an Landplanarien," 'Inaug.-Diss.,' 
Leipzig (l)erliii), pp. 1 — 47. Also in 'Arcli. f. Naturgescli.,' 57th 
Jalirg., pp. 306—350. 

LiLLiE, l'\ R. : 00. — "Some Notes on Ilegeneration and Regulation in 
Pianarians : I, The Sonrce of Material of New Parts and Limits of 
Size," ' Amer. Nat.,' vol, xxxiv, pp. 173 — 177. 

LiLLiE, V. \i. ■ 01. — "Notes on Regeneration and Regulation in Pianarians," 
'Amer. Journ. Physiol.,' vol. vi, pp. 129 — 141. 

LoEB, J. '93. — " Uebcr kiinstliche Unwandlung positiv heliotropischer 

Tliiere in negativ lieliotropische uiid umgekehrt," 'Arch. f. d. ges. 

Physiol.,' Bd. liv, pp. 81—107. 
LoEB, J. '94. — "Beitriige zur Gehirnphjsiologie der Wiirmci'," ibid., 

Bd. Ixi, pp. 247—209. 
LoEB, J. : 00. — ' Com])arative Physiology of the Brain and Comparative 

Psychology,' New Yorl< (G. P. Putnam's Sons), 1900, pp. x and 309. 

LoEB, J., AND Maxwell, S. S. '90. — " Zur Theorie des Galvanotroiiismu?," 

' Arch. f. d. ges. Piiysiol.,' Bd. Ixiii, pp. 121—144, Taf. i. 
LuDLOFF, K. '95. — " Untersuchungen iiber den Galvanotropismus," 'Arch. 

f. d.ges. Physiol.,' Bd. lix, pp. 525—554. 
MiTSUKURi, K. : 01. — " Negative Piiototaxis and other Properties of Litto- 

rina as Factors in determining its Habitat," ' Annot. Zool. Japon.,' 

vol. iv, pp. 1 — 18. 
Morgan, T. H. *9S — " Experimental Studies of the Regeneration of 

Planaria maculata," 'Arch. f. Kntwickelungsmechanik,' Bd. vii, 

pp. 304—397. 

Morgan, T. H. : 00. — "Regeneration in Pianarians," ibid., Ed. x, pp 
58—119. 

Morgan, T. H. : 01. — "Growth and Regeneration in Planaria lugubris," 
ibid., Bd.xiii, pp. 179-212. 

Morgan, T. H. =02. — " The Reflexes connected with Autotomy in the 
Hermit Crab," ' Amer. Journ. Physiol.,' vol. vi, pp. 278—282. 

MosELEY, H. N. '74. — " On the Anatomy and Histology of the Land 
Pianarians of Ceylon ; wiih some Account of their Habits, and a Descri))- 
tion of Two New Species, and with Notes on the Anatomy of some 
European Aquatic Species," ' Pliil. Trans. Roy. Soc. London,' 1874, 
pp. 105—171, pis. x— XV. 

Moseley, H. N. '77. — " Notes on the Structure of Several Forms of Land 
Pianarians, with a Description of Two New Genera and Several New 
Species, and a List of all Species at Present known," ' Quart. Journ. 
Micr. Sci.,' N.S., vol. xvii, pp. 273—292. 



MOVEMENTS, ETC., OF FliESH-WATKK PLANAllIANS. 713 

NoKMAN, W. W. : 00.-" Do ihe Reactions of ti.e Lower Animals a.-ainst 
Injury indicate Pain Sensations?" ' Amer. Journ. Pliysiol ' vol iii 
pp. 271— 284. J •. • , 

Parker, G. II., and Burnett, F. L. : 00.-" Tlie Roaclio.is of I'lanarians, 
witii and williout Eyes, to Li.^lii," ibid., vol. iv, pp. 373—385. 

Patten, W. '<JG.-" Variations in ti.e Development of Limulus poly- 
piiemus," 'Journ. Morpli.,' vol. xii, pp. 17—148, pis. ii— xl. 

Pearl, R. : 00.—" Studies on Electrotaxis : I, On ti.e Reactions of certain 
Infusoria to tlic Electric Current," ' Amcr. Journ. Pliysiol., vol iv no 
9G— 123. • -ll- 

Pearl, R. : 01.— "Studies on the Effects of Electricity on Organisms: 
II, The Reactions of Hydra to the Constant Current," ibid'' vol v 
pp. 301— 320. ■' ■ ' 

Preyer, W. 'SG.— " Ober die Bewcguii-cn der Seesterne : ein veri,rlcichend 

physiologisch-psychologischc Untersucliung," Erste Hillfte, 'Mittli. a. 

d. Zool. Stat, zu Neapel,' Ed. vii, pp. 27—127. 
Preyer, W. '87.—" Uber die Bewegungen der Seesterne : ein vergleichend 

physiologisch-psychologische Untersuchung," Zvveite Halftl-, ibid 

Bd. vii, pp. 191—233, Taf. vii. 

Rasfail, X. "93.— "Note preliminaire sur une Planaire, sp, ? " 'Bull. Soc. 
Zool. France,' t. xviii, pp. 49, 50. 

ScouRFiELD, D. J. '91.—" Entomostraca and the Surface Film of Water," 

* Journ. Linn. Soc. Zool.,' vol. xxv, pp. 1 — 19. 
ScouRFiELD, D. J. : 00.— "Note on Scapholeberis mucronataand the 

Surface Film of Water," ' Journ. Quekett Micr. Club,' Scries 2 vol vii 

pp. 309-312. 

SCOUKFIELD, D.J. : 01.— "Hydra and the Surface Film of Water." ibid.. 
Series 2, vol. viii, pp. 137—112, pi. viii. 

Steiner, J. '98.— 'Die Functioncu des Centralnervensystems und ilirc 
Phylogenese: Dritte Abtli.,' "Die wirbelloseu Thicre," Braunschweig 
(Vieweg und Solin), 1898, pp. x and 151., Taf. i. 

Uexkull, J. VON. '9G.— "Ueber ReQexe bei den Seeigeln," 'Zeitschr. f. 
Biol.,' Bd. xxxiv, pp. 298-318. 

Uexkull, J. von. '9G, «.— " Vergleichendc sinnesphysiologische Unter- 
suchuiigeii: II, Die Schatten als Reiz fiir Ceii trostepha nus longi- 
spinus," ' Zeitschr. f. Biol.,' Bd. xxxiv, i)p. 319—339, Taf. i— iii. 

Uexkull, J. von, '99.— "Die Physiologic der Pedicellarien," ibid.. Bd. 
xxxvii, pp. 33J;— 403, Taf. iv, v. 

Uexkull, J. von. : 00.—" Die Physiologic des Seeigelstachels," ibid., 
Bd. xxxlx, pp. 73 — 112. 



714 EAYMOND PEARL. 

Uexkull, J. VON. :00, a. — "Die Wirkuiig von Licht und Sclialten auf die 

Seeigel," ibid., Bd. xl, pp. 446—476, Taf. i. 
Verworn, M. '89. — 'Pfeycliopiiysiologisclie Protibleiisludien,' Jena (Fi&hei), 

18S9, pp. viii and 218, Taf. i— vi. 
Whitman, C. O. '99. — "Animal Behaviour," 'Woods Holi Lectures for 

1898,' i)p. 285—338. 

WiLLEY, A., '97. — " On Heteroplana, a New Genus of Planarians," ' Quart. 
Jouni.Micr. Sci.,' N.S.,vol. xi, pp. 203—205. 

Wilson, C. B. : 00. — "The Habits and Early Development of Cere- 
bratulub lacteus (Verrill): a Contribution to Physiological Mor- 
phology," 'Quart. Journ. Micr. Sci.,' N. S., vol. xiiii, pp. 97—198, 
j)ls. ix — xi. 

WoODWORTH, W. McM. •91. — "Contributions to the Morphology of the 
Turbellaria : I, On the Structure of Phagocata gracilis, Leidy," 
' Bull. Mus. Cump. Zool.,' Harvard College, vol. xxi. No. 1, pp. 1 — 42, 
pis. i — iv. 

WooDWORTJi, W. McM. '97. — " Contributions to the Morphology of the 
Turbellaria: II, On some Turbellaria from Illinois," ibid., vol. xxxi, 
pp. 1 — 16, pi. i. 

Yerkes, R. M. : 02. — " A Contribution to the Physiology of the Nervous 
System of the Medusa Gonionemus Murbachii: Part I, The 
Sensory Reactions of Gonionemus," ' Auicr. Journ. Physiol.,' vol. vi, 
pp. 434—449. 

Zacharias, O. '88. — " Landplanarien auf Pilzen," 'Biol. Centralb.,' Bd. 
viii, pp. 542, 543.